I'm drawing a big blank trying to figure this out.
I am wanting to install a rail over a tapered barrel and need the rail to to be on the bore axis so I can use flip up AR type sights along with a red dot.

The rail will be on top of the receiver, which is larger than the muzzle end of the barrel (duh). What I need to know is how to figure the needed height of the block at the muzzle for the rail to mount.

Here's a rough drawing of what I'm wanting to do.