Bob,
Good technique learned from Bill Raby is to take your barrel and using a Sharpie or Prussian Blue, darken both of your barrel side flats. Scribe a line for 1/2 of the flat or less, depending on the school you are building. Start by marking the middle of the flat at the breech and then again at the muzzle. Now lay the straight edge on those two marks, clamp the straightedge in place and scribe your line on the side flat. This line, when the barrel is placed back n your barrel inlet, will guide you where to bring your forearm wood down to. Seems to be a pretty good technique. Interesting point, on an original I am studying, that lime is not nearly as straight as we can do with our modern measuring and marking tools. Good luck and I’m sure you will do a great job!
elkhorne