My calculation was specifically relating to the "being up in a tree 14 feet".

To truly calculate where the bullet would strike at 20 yards you would need to know how high the scope was mounted above the barrel and what the bullet trajectory is.

Since the scope is above the barrel, at the initial firing of the bullet, it actually climbs rather than falls since you have your barrel pointed just a tiny fraction up. Generally it peaks in the 50 - 60 yard range and then would start falling again to get to the targeted sight in range of 100 yards.

Based on my limited knowledge of shotgun sabot ballistics I would think that at any point in that 100 yard trajectory, the bullet would never be more than 2" from it's aiming point. As it exited the barrel it would be 2" below the aiming point, then it typically crosses the aiming point on it's way up at about 25 yards, then rises to about 2" above the aiming point at 50 - 60 yards then drops back down to the arrive at the aiming point at 100 yards. It might not be that much drop though, I'm not sure on a shotgun ballistics.

Hence, the common suggestion here of taking dead aim and pulling the trigger. You'll never be off more than 2" high or low and at 20 yards you should be almost exactly on your aiming point.