The position of the rear entry relative to the breech is proportional to the length of the barrel, or at least, that is the way I determine the position. I layout the entire gun on the stock blank with the actual parts before I cut it and the rear entry is one of the things I mark. I start by positioning it 1/3 the length of the barrel from the breech. Then I look at the position of the barrel tennons and thimbles. I try to position those roughly equidistant from each other and out of the way of the barrel tennons. The postion of the barrel tennons take precedence. You don't want a tennon at the waist of the barrel and the position of the first tennon depends on the region and period the rifle will represent. Once, you have all that worked out, I make sure that the upper/lower forearm transition is at least a hands width from where the rear sight will go. I do not consider balance as I don't think that was ever considered by any of the original builders. Most original guns, at least the ones I build were extremely muzzle heavy, intended to be shot from a rest, and carried on a horse. That said, doing all the above and using a swamped barrel, the balance point is generally within a hand width of the transition. Oh, and one last thing, I step back and ask myself if it looks right. If not, I move it. In the final analysis, I go with looks right over a lot of the other factors.
I hope all that is clear, but I suspect not. I agonized over this stuff for years before getting comfortable with the above procedure.
Mark