You might want to consider a couple of things after you determine your pull using the "arm adjacent to the elbow to bent index finger".
Things that come to mind are How is the gun going to be used?
If it is short sleeve style target shooting, the above method works pretty well, but, if it is for deer hunting while bundled in a heavy coat and with heavy gloves on, reducing the "pull" an inch less than the measured distance can be a good thing.
Another consideration is the type and length of the barrel.
If it is a straight octagon barrel, 42 inches (or more) long, it will be quite muzzle heavy.
Shorter pull lengths put the weight closer to your body so it won't feel quite so muzzle heavy.
Longer pulls put some shooters more at ease when shooting a Flintlock. Seems some folks don't like the pan flash real close to their bushy beard. :grin:
Seriously, some people don't really like the "blast" they can feel with their faces when the pan flashes.