Probably the easiest way to see if your saddle fits right is to ride the horse all tacked up for say, 20 to 45 min. When you take the saddle & pad off - everywhere the saddle & blanket touched should be sweaty. It's not a perfect way to see, but it's a round about way. If there's dry spots, you don't have contact there, and presumably, the areas immediately around it are pressure points from the saddle. Otherwise, you should be able to slip your hand all around the saddle when it's on & cinched, and you should be able to get your fist between the withers & the gullet. Longterm - you'll know if there is a major pressure point somewhere if you ride a LOT over the summer, and you find spots of white hair on his back after months of riding. Until then, if you are still questioning the fit after you've done all that, I'd take horse & tack to a saddle shop and have a 'pro' check it out. Most good saddle shops will do this free of charge. Good luck :)
As a side note, I personally take issue against flextree saddles. The on the surface idea is great, but cowboy logic says that trees were designed to distribute the weight of the rider. A flex tree goes against that. It still distributes some of the weight, but not like a regular old western tree. If someone disagrees with me, that's fine, but of the people that I know that ride a LOT, the few that switched to a flex had lots of trouble with chiropractic issues with the horse within a few years. I don't think it's an issue if you don't ride heavy though.(I mean several hours a day, 6-7 days a week here.) Just MHO.
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