Insert a tight fitting dowel into your bore. Lay your rifle on a piece of packing paper. Draw a line at the top of the dowel. Make a horizontal tick mark where your rear sight will be and a vertical tick mark where your front sight will be. Remove your rifle. Extend the dowel line just past the rear sight mark. Then extend the rear sight mark parallel to the dowel line until it crosses your vertical front sight mark. You now have all the informatton needed to make your sight measurements. Ce n'est pas?
For all the guys who are big on making a set of exact blueprints before they build a gun, of which I am not one, this should have already been done.
**in retrospect, you can do it with the barrel alone if that is more convenient.