I had a hard time getting an accurate measurement until some one posted a nifty procedure.
Let's see if I get this right /ubbthreads/images/graemlins/blush.gif
Distance from bottome of barrel to top of scope front bell (at the front of the scope. Subtract 1/2 scope front bell diameter and 1/2 barrel diameter at the front of the scope bell.
Turns out this is much more accurate for me than messing w/center line of bore to center line of the scope.
HTH