There are a pile of factors that affect the accuracy of reticle ranging. I looked at your reticle on Vortex's website, and one of the limiting factors is the 2moa interval of your grid. The thin lines are 0.15 moa wide, but you don't have anything definitive in between those two measurements. Judging one moa is do-able pretty easily, but for most trajectories that interval is too coarse for ranges 500 yards. At my club we have steel targets of various shapes at each berm, from 200 to 500 and the farthest is 640 (behind the competition target pit). For practice, I make my most careful measurement of each plate, then when I go to paint them I'll get out the tape and see how close I was able to come to the truth. It is a humbling exercise. My HSLR has both 1moa and 0.5moa hashes, which improve my chances. Your tactical reticle is intended for quick ranging of man-size targets, which are taller kill zones than most game animals, hence more forgiving. You said in your OP that the target is 36 inches tall- if there was any angle aspect to your line of sight, this may have skewed your measurement, like when looking up or downhill at a deer makes the body look smaller, leading to an over-estimation of range. When scoping a target of known dimension, you must account for any perspective angle and use the least distorted dimension. And one more note on FFP: the one I use has an open center cross with a 0.15moa dot. A target has to be pretty small to be obscured by that.
