How would the 18β fit if you wanted to bring your stirrups up a hole or two?
My rep brought a bunch of different models and sizes of saddles and had me sit in them without knowing anything about them and had me close my eyes and feel which felt best then we tried those on the horse and came to some conclusions. Fwiw, I also have long legs-particularly femurs- and I ended up in a 18.5 with an extra forward flap. Iβm 5β8 and 125. Go with what feels right for both of you. And if in doubt, shorten your stirrups, most ride far too long to be properly balanced