...All based on the mil-dot mil-ranging formula (even downrange zeroing with any multi-stadia reticle and/or turret is based on this simple equation). Here it is--
Rangefinding (inches to yards)-- target size (") x range of reticle subtension measurement (usually 100 yards) / reticle subtension (") / quantity of gap tgt. occupies (decimal equivalent) = range (yds.)
Example-- Leupold Duplex reticle used for their range estimating system is designed for a 16" deer back to brisket...but the mil-ranging formula is better than their system as designed. Pg. 100 of '10 catalog shows the subtension as 5.4 inch per 100 yds. between plex post tips @ 14x. 16" b-b deer brackets perfectly between post tips (100% or 1.0 decimal equivalent). Now punch figures into equation--
16 x 100 / 5.4 / 1.0 = 296 yds.
Same tgt. occupies 1/2 of the gap (0.5)--
1600/5.4/0.5=593 yds.
It's not always this easy though as RES's are based on the shooter's ability to interpolate between stadia points accurately and guess tgt. size accurately, which gets geometrically more difficult the further out the tgt. gets, and some other variables.
