It really doesn't matter what you use as zero, so long as you know the trajectory of your load and have a reasonably accurate estimate of the distance when you shoot at game. It's not all that difficult to place your sight center of chest, or at the top of the back, or six inches over the back.

But you must know your actual trajectory. I do a "trajectory study" with every gun and hunting load and keep the record. Shoot three shots each at 25, 50, 75, 100, 125, 150 and 175 yards. Then you know two things for sure; (1) the limit of your practical hunting accuracy, and (2) where to place your sights for any particular shot.