Yes u can use the stadia in a reticle to measure the distance to a tgt. of KNOWN dimension. The best way to accomplish this is to apply the "modified mil-ranging formula." All rangefinding with reticles (stadiametric rangefinding) is based on this simplified equation. Here it is (inches to yds.)--
tgt. size (") x range of reticle subtension (usually 100 yds.) / reticle subtension (") / quantity of gap tgt. occupies between stadia (decimal equivalent) = range (yds.)
...looks complicated, but quite simple to apply--
Example--sounds like u have the NP-R1 reticle (maybe R2?). If it's the R1 then i think the stadia to stadia gap is 1 inch per 100 yds. in that reticle (better check that 1--don't know for sure). Suppose u look at your big bull elk at an unknown distance and u see that he occupies 3 and 1/4 of the 1 IPHY stadia units. Most folks say that the avg. bull elk is 25" back to brisket, but this guys a biggie so lets give him 2 more inches at 27. Now just fill in the variables in the equation--
27 x 100 / 1.0 / 3.25 = 830 yds.
...BUT...it's not really that easy oftentimes. Obviously u need to guess the tgt. size correctly, and guess the "gap" correctly too.
Now let's see how far off u will be if u're off in tgt. size by only 1"--
so as u can see +/-1" gives 60 yds. variation in range estimation, and thats assuming u've guessed the gap accurately to a level of 1/20th of it's subtension (.05).
It should be obvious that as range increases the error increases geometrically.
I've found reticle rangefinding with scopes is fairly accurate to 400-500 yds. and then drops off dramatically beyond that.