You know Ian, i've never worked with a flat-top post, as seems most of the more common plex posts are pointed. I've only used this method myself once with my 700 6AI. Had a 4-12X Burris Mini on top. Exbal calculated the plex zero, and i had a Stoney Point target turret screwed on. I can't remember offhand what the post was zeroed for, but i ranged a little cactus on a teepee butte at 600 some (been awhile-- can't remember exactly). Ran the clicks in, got steady and shot, and darn if i didn't nail the cactus. (even saw the hole in it). I was coyote hunting at the time-- sure wish it was one of them-- oh well, next time. I know this has nothing to do with ranging with the reticle, but it was fun nevertheless. This guy I was speaking of has got quite a shooting savvy, and has made the system work on several elk and antelope, as well as coyotes. I've been working with the Burris Ballistic Plex reticles myself recently, and that's been a bunch of fun, especially in combination with Exbal.
I've ben using duplex reticles for range estimation for many years. I do it a little differently than the other posts on this thread; for many years I have kept a record of the back-brisket heights of animals. Based on these heights at 100 yds, I have a yardstick set up with these heights divided by 1/4, and set up the yardstick in my back yard at 25 yds (as opposed to 100). When calibrating a scope for AZ whitetail, for instance, which average about 14" for a good buck, I calibrate the scope to the 3 1/2" mark (14" divided by 1/4) by turning the power adjustment ring until the duplex steps just cover. I then record the power.
Obviously, if the deer on the hoof extends from step to step it's 100 yards away; if it extends from a step to the crosshairs it is 200 yard away, etc. Using this method, I can pretty accurately range to about 500 yards; then it gets a little shakey. Not as good as a rangefinder for the long shots, but faster. Also, not real good for a deer other than broadside.
I have 2 Leupold 2 1/2-8's and the duplex extentions are quite a bit different! The first was about right for elk, covering 24" at 2.6 power, but the second one couldn't be turned down enough to cover the 24". Also, don't assume that if a scope covers 12" at 6 power that it will cover 24" at 3 power; they do not have straight-line advances!
This system has never failed me, but I failed it once because I just didn't believe the scope. In the reticle the deer appeared to be in excess of 500 yards and I refused to believe that it was over 400. I held a little over its back and hit it in the knee! I eventually got it, but it took some chasing. So much for the 8 deer with 8 shots record of my 25-06.
I have done this for years. Animals that live in areas with little background cover for a good laser lock like antelope and caribou render laser rangefinders useless. I set the average of my mid range trajectory the same as the back to brisket measurement of my quarry. For instance caribou average about 21". When I zero my rifle at 650 yards the mid range trajectory is within 3.5" up or down of 21" from 175 to 575 yards. Simply adjust the power ring to frame the caribou between the top duplex and the crosshare which measures 21" for you. Now the point of your top duplex is 21" high at the point where the caribou is standing. Hold that point dead center of the chest and shoot. If the caribou is between 175 and 575 you will hit not more than 3.5" high or low, or in other words very dead caribou. If he is further than 575 then you have two other sights to use which are the crosshare and bottom duplex. I know exactly where these hit also. In hunting situations many times you don't have time to turn clicks and this method is very fast. I have been doing this for thirty years and everyone that hunts with me are amazed how I never miss at long range. This method works very well out to the limit of where I shoot at game which is about 850 yards. Usually I can figure a stalk to get within this range. Just keep in mind things that can decieve the system like steep up or down shots.
Welcome Oldtimer. You say this system has been working for you to 800+ yds.?? Taht's excellent. I would have thought that resolving power of the scope as well as environmental conditions would put a shorter limit on this "optical" rangefinding system. But if you say you're going out that far then now you've got me thinking. Sometimes i get in trouble when i do that.
Had a question about the use of plex reticle for second zero reference from Vic in MT (sorry Vic had a computer or maybe computer user glitch and lost it). Here is what i've come up with from my research. I've created a plex reticle worksheet that can be used to calculate a rangefinding system and additional zero's for any plex reticle. At exactly 102 yds, from the kitchen table out my front window is a sign post with holes along it that measure exactly 1" apart. This provides a quick and simple measurement of the reticle subtension. When a friend comes over we set his gun up on bags on the kitchen table ( when the wife's not home) to measure the reticle (at the scopes highest mag.). As soon as i obtain the mearurement i write it on a sticker and put it on the inside of the turret cover for future reference if needed. If he knows the m.v. of his load i then follow the following calculations to maximize the potential of the reticle.
1) Run the loads ballistics figures through Exbal (or any other ballistics program) to determine the maximum point blank range (MPBR) of the intended target.
2) Then calculate the mag. needed for MPBR for rangefinding purposes using the following LINEAR formula: highest scope mag./ target measurement= X mag./reticle subtension @ highest scope mag. @ MPBR. Now this means that when you adjust the scope to this calculated (X) mag. when the target subtends the reticle or is bigger, just aim dead center and shoot for a certain (theoretically) hit. Now here's the nice little additional feature that one may wish to adopt. Suppose the rangefinding mag. needed on a 4-12X scope is 5.5X. well i don't know about you guys but when i'm presented with the opportunity (time actually) to use this system, i want the highest mag. i can get for a calculated shot-- right? So instead of using the center x-hair- post gap for rangefinding try using 2X that mearurement (11X in this case) for the post-post gap instead. This should provide a higher mag. for rangefinding purposes, as well as additional reference (the center x-hair) for approximating range at longer distances.
3) Now in order to calculate the new 100 yd. subtensions for bottom post zero @ MPBR mag. apply the following formula: X" @ 100 yds. @ MPBR mag./highest scope mag.= subtension @ highest scope mag. @ 100 yds./scope mag. @ MPBR. From here i use Exbal to calculate the new zero for the lower post (at the mag. needed for MPBR rangefinding). I can also punch in 2X that measurement for an aditional zero (in this case suppose that zero range is 625 yds. If i've calculated this distance to the target simply hold the lower post on the target, note where the x-hair is then bring the lower post to that spot and shoot--- theoretically, mind you). trying to create a mathematical model for this calculation is no longer linear and requires algebraic formulas since the bullet trajectory is parabolic in nature.
Of course, this is all theoretical since i just thunk it up recently, but it should work (check my calculations please administrators) since the relationship between reticle subtension, apparent image size and scope magnification is linear, and should at least get you on paper, and while you're testing this mathematical model it might be a good idea to check to make sure the MPBR for rangefinding is right also. The nice thing about this system is that it can save time and wasted bullets at the range.
Sounds like Old Timer's got the system down to a science as well-- even more since he appears to be using his upper post as the original zero for MPBR.
Ian, Bob Hagel's book, "Game Loads + PracticalBallistics for the American Hunter" has published measurements for buck antelope and bull elk(Rocky Mtn. i assume). Bull= 32-34" back-brisket (b-b), vital area= 22"; buck= 15-16" b-b, vital=10". Obviously approximate figures.