I don't try to calculate the distance prior to shooting. Instead, I try to determine whether I think I can hit the animal lethally with a high degree of certainty. That usually means my shots will be less than 30 yards, with most of them significantly less than that. I still think the best way to practice for hunting is by shooting at various distances between a point as far away as I can consistently hit and a point just a few yards away from me. For me, trying to think of the distance to the target, especially when hunting, is a recipe for disaster.
Allan