Apparently there are at least two mathematical methods for using Mil-dots. Since I have trouble counting my fingers I have to keep this all very simple.
The base formula that I was taught is
Size of Object in inches times 27.77 divided by the Mil reading. That is a bunch of frigging math. But, there is a simpler way that does work well in the field.
Let's use the magic 18 inches, which is about the backline-brisket dimension of a good buck, or the ear width of a Sask. whitetail. 18 times 27.77 rounds off to 500 (499.86). That is a constant. So you estimate the Mil reading in tenths and divide that number into 500. Result is distance in yards. If it is exactly one Mil the distance is 500 divided by one or 500 yards. Two mils would by 250, half a mil would be 1000.
You can figure these constants for any numbers, 10, 12, 15, 18, 24 etc. any number that relates to the critter that you are hunting. When you know a good working number create a simple chart for that constant and what Mil reading would be what distance and fix it to the side of your rifle.
The ugly challenge in using Mil-dots is learning to mentally estimate 1/10's. You have to mentally break each Mil into 1/10's.
I was very fortunate to be taught by Steve Suttles and Bobby Whittington (Badlands Tactical Training) - these guys live this stuff and they were very patient as they spent a couple of days on it so that I could really understand their system.
They use the big posts as a starting point for Mil-ing, rather than the crosshair intersection and that sure works for me. They also had me use a pocket calculator continually so that I got used to the frigging math. I believe that I introduced them to the Mil-dot Master and they are now using it, it is a great tool
How in hell does one decide 3 tenths from 4 tenths? One way is to go out in the country and set up some 18" boxes or pieces of cardboard, plywood or whatever. Move back a good distance, lay down, set a post on one side of the target and estimate how many tenths of a Mil the box is - or it might be one Mil plus 3 tenths. Do the math - 18 times 27.77 divided by the Mil reading or simply divide 500 by the Mil reading and you have a distance. Record that number and then do the following.
First, laser or pace off the distance so that you know what you are working with. When you know the distance you can plug it into the formula to determine what the correct Mil reading should have been. Let's say it was 600 yards so the reversed formula would require simply dividing 600 by 500 = 1.2 Mils.
Much simpler is to buy a Mil-dot Master. Then you can do the above very simply. You know the distance is 600 yards so you move the slider to line 600 yards with the indicator on the left column. Then in the right hand column you look at 18 and check where the number 18 lines up with the Mils which are on the far right. This let's you find out what the actual Mil-reading should be so that you can start getting a good mental picture of each tenths location. This is much simpler to do than it reads.
No-one in their right mind uses Mil-dots when they could be using a laser for hunting. But if the battery is dead or the damn laser won't give a reading the Mil-dot will give a better distance than a wild-ass guess. That is if you know the size of the object and how to breakdown 1/10ths. Not knowing these distances turns the dots into little black things on the crosshairs. The use of CONSTANTS is a great idea.
Good luck with the dots. Two othe points - Mil-dots do provide a great set of constant hold-off points. The new Nikon tactical comes with .5 hashmarks.