Have you done a proper wither tracing and carried around the template of that tracing when you went saddle hunting?
Many companies that make saddles, have wonderful "How-To" guides on the proper way to measure your horses back and withers.
In some cases the only way to get a proper fit, is to call out a saddle fitter and then special order a saddle. If you have to do that, make sure you order from some one who excepts the saddle back for adjustments, etc. as your horse will most likely change shape over time.
It can get rather expensive in the long wrong, but at least you know that your saddle will fit and be of top quality.
It's a lot like nuts and bolts - if the rider's nuts, the horse bolts! ~Nicholas Evans