Not too difficult at all. You want half of the side flat to show above the wood, so take the total distance from bottom to top flats. Divide it in half - that will be the total depth of the inlet at that point. As it is tapered, simply repeat at several spots along the barrel. Note that some schools of building will want more or less of the barrel to show; for a Lehigh, as example, 2/3 of the side flat will show.
If it is a swamped barrel, from the breech forward it will taper smaller, then a little shy of the muzzle it will start to widen out again. Again, measure at several points. You can use a fine point pencil, make a mark on the side of the barrel inlet, and write down what the depth will be at that point.
Good luck on your build - keep us posted.