You are in Idaho? If you are interested in the mountain man era then you'd want to add that into the equation. I might be wrong but from some of the paintings, plus bags shown in Hanson's "The Plains Rifle" I get the notion that the bags carried by mountain men might have been a little larger than the eastern bags. The eastern bags might have been carried by a frontiersman living in a log cabin and out for a few days and therefore, it carried a few days supplies. The mountain men might have carried more in their bags and used larger bags.
I think a lot of original bags were "sewn flat". That's the way it was. Fold over the leather and sew the edge. These sewn flat bags don't seem to provide that much interior space so now folks sew bags inside out or add gussets, etc. On the original bags, if soft brain tan was used (NDN made) there might have been a few where a flat front panel was slightly smaller than the back panel and this created sort of the same effect as the "puckered toe" on a moccasin. I'm not certain how common gussets were on original bags. In any event, the exterior dimensions depend a lot on the design, the "sewn flat" types need to be larger than the inside out or gusset bags.
I always make a proto-type from cheap vinyl until I have what I want and then use the vinyl as a pattern for the leather.