I’m confused by how changing stirrup length affects the fit of the seat? I can see how it would affect how the flap size/length/placement works for you, and I understand how something like depth of seat affects what length stirrup you’re most comfortable riding in, but I don’t understand how shortening or lengthening the stirrups changes the fit of the seat.
Flatwork length, XC length, galloping or doing sitting trot work, my balance point may change, but the seat area I need doesn’t, because my body remains the same size. When I’ve tried saddles (and I tried a bunch the other year) the first thing I did was get on and walk around without stirrups to see how they felt, and I could usually tell right away whether they worked for me. It took more than that to see how they worked for the horse, usually, but for a person it should be relatively easy to feel.