rule of thumb i have heard often is a mature whitetail is 18" when measured vertically from base of back of neck at shoulder blades to chest hairs on the bottom... with a vital zone of 8 inches. i'd be hesitant to risk a range estimation with mil reticle at anything past 400 yards, given bullet drop past 400... but i think a mil reticle could provide a very fast range estimate, good enough, within 400 yards... i need to stop thinking this and just get out and shoot.