All of those systems of measurement are based on the same mathematical formula that is used by the mil-dot guys to range a tgt. Here's the most basic subtension ranging equation (inches to yds.)--
tgt. size (") x range of reticle subtension measurement (usually 100 yds.) / reticle subtension (") / quantity of the "subtension unit" tgt. occupies (tenths of the total subtension) = range (yds.)
Looks complicated, super-simple to apply. Suppose the subtension of your plex post tip is 5 inches @ 100 yds. post tip to post tip, and suppose u want to set up a system to range a 16" deer back to brisket (Leupold's std.). Just substitute the variables into the equation above to get this--
16 x 100 / 5 / 1.0 (deer brackets x-hair to post tip perfectly @ 1.0 subtension units) = 320 yds.
Now recognizing that 320 is the tgt. size vs. reticle subtension constant, simply put it into the calculators memory, and finish calculating the ranging chart for that tgt. size--
320 / 0.9 = 360
320/.8 = 400
.7=460
.6=530
.5 (x-hair) = 640
Couple things should be noted here. The further out u go the less accurate the system is, and it all depends on how well u can interpolate between stadia (takes some practice), and if tgt. size changes then that also introduces error. When the mil-dot guys apply the equation their reticle subtension constant is 27.8 (100 yds. / 3.6" = 27.8).
Edited by sscoyote