5/16" is way, way, way too thick. I drill a quarter inch "witness hole" to see where the rod is. Can't really tell what's goin' on with a tiny little hole. If the hole is bigger, you can tell exactly where the rod is. If it's off center, you'll get a false reading with a tiny hole and a piece of wire as a feeler. The hole is easily plugged later, if you wish.
Depending upon the barrel and what I'm going for, I like about 1/8th inch for a web right at the breech (with the barrel taper, it will be a hair more at the front lock bolt). And usually about 3/16" at the muzzle, again, depending upon the barrel.
Are you using the Chambers "Early Germanic" lock? I have always found this lock problematic in proper positioning, as the nose of the lockplate is pretty low, so the lock and front bolt have to be taken into account when positioning.