No matter how much your horse grows/muscles out, the whithers will remain narrow and high. You may have a little widening, but not much.
The saddle honestly should naturally "fall" into place...as in you can shift it on the horses back and there will be a spot that naturally "feels right."
When tacked and girthed, you should be able to get your fingers between the pommel and your horse's whithers...if you can't, you need a tighter tree or you can also try a built-up cut-back pad if it only needs a little raising.
Both my boys were a nightmare to fit...Hoover takes a semi quarter (6.5) and Gun takes a regular (6").
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux