I'm a bit iffy on TB type horses. The one that we used to have and I rode was a thicker bodied guy, but he had a relatively small girth area for his size. I used a 28 on him but probably could have gone with a 26.
But on the other hand, I rode a customer horse not too long ago (an appendix mare, looked very TB) that, at first glance, looked like she would be great in a 28. Yeaaahhh, she ended up needing a 34
Did you cinch up the new girth you tried out relatively tight? Do you remember about how much space was there between the cinch buckles and the rigging on your saddle? Ideally, there should be about 8 inches, though for general riding, 6 is acceptable on each side. If you can remember about how far it was, you can just subtract the difference and voilą, you have your measurement