Or: measure the thickness of the tang right behind the guard/bolsters, whatever, (i.e. the thickest part) then measure the thickness of the tang at the butt. (i.e. the thinnest part) then divide the difference by two. This is the amount you should shim the end of each scale at the butt end off of the drill press table when drilling so that the angle through the pin holes is straight when installed. So if the tang is 3/16" at the thickest part, and 1/16" at the thinnest, the difference of 1/8" total when split in half is 1/16" per side. When you drill, shim the end of the scale that is planned to be the butt, up off of the drill table with anything, even a drill bit, that's pretty close to 1/16" in thickness. Do this on each side, install the scales, and you should be able to slide a snug fitting pin all the way through without major difficulty and without affecting the fit of the scales. I often do it this way since I often forge taper into the tang before getting the chance to drill anything.