Originally Posted by Conor
1.3" is still 1.3" no matter what magnification you set your scope at??
My guess is, that he means 1.3 mildots not 1.3 inches.
In that case my best guess at the problem would be that the mag on your scope isn't at it's true mildot setting.
btw, to get accurate results (although it won't matter in this case) you do need to enter the correct line of sight above muzzle value (it's NOT scope height). The best and most accurate way to do that is the tin foil method.
@Jack - Simple trig unfortunately will not work because there are some unknowns that you erm.. don't know