Don't confuse shooting for POI with gun fit. When you're shooting to check POI you want to know where the gun shoots by removing the influence your mount and dimensions have. The best way to do this is to take two quarters and rest them on top of each other. Place them at the rear of the rib just in front of the breech. Aim at a spot on the board by placing the bead so it sits atop the two quarters. You've essentially created a rear sight for the gun. Shoot it like you would a rifle. This will tell you were the gun is shooting and let you be confident with applying your measurements.
It's possible that you may shoot a gun well that doesn't fit. I've found out that this tends to happen when the gun doesn't shoot where it's supposed too and it just so happens that the poor fit compensates for it. For example, the gun is too high for you but the POI is low which essentially cancel each other out.
It's much better to determine POI and then worry about measurements. You may be bale to correct for POI problems by using different dimensions than what the try gun says you should use.
