I use a sort of similar procedure, but place the barrel in the stock at 90 degrees, then mark a vertical line on the side of the stock at each point where a hole will be drilled. I then place the barrel on the table of the press against a board ( or other 90 degree fixture) clamped to the table and adjust it until the point of the drill will contact the place on the vertical line where the hole should be ( or in the case of barrel loops where the drill will just clear the bottom of the hole in the loop. Then place the barrel in the stock, and clamp barrel and stock and board all together with drill on the vertical line. This way the drill is positioned both vertically and laterally in the proper place . So far this has worked well and is simple and accurate if all are positioned securely and carefully. I have done this on both squared up stocks, and also on shaped stocks IF THE LOCK PANELS ARE SQUARED WITH THE TOP BARREL FLAT ( a sort of small but important point. ). If not square the hole will still hit where it should but will be higher ( or lower) on one or the other sides. (Still functional, but sort of cosmetically like shaving your sideburns when drunk.)
I remain, respectfully, your Hoosier example of muzzleloading’s Occam’Razor. Mikeyfirelock