On the thick butted (2" to 2 1/8") rifles I build, I use 1/4" castoff (on both the top and bottom). The pivot point on top is the center of the rear of the tang. On the bottom, it's the center of the rear of the trigger. These points do NOT line up. This doesn't really matter, though, because of all the blending, as Jim Kebler points out. I set the trigger and front finial of the guard on the centerline of the barrel, and the rear finial along the castoff line, which means the guard bow is askew of the trigger. Nobody will notice it. Nonbuilders don't even notice the castoff until you point it out to them.
Andy