Absolutely best if you can try the saddle beforehand, as a poorly fitted saddle can cause all manner of problems.
That said, Georgie and I really like his Synergist endurance saddle, and he was tough to fit.
They're kinda pricey new, but for a modest fee, they will rework a used Synergist saddle to fit your horse (within reason). They use something called an Equifit (something like that), which is a sheet of special plastic that you heat and mold to your animals back. This is then used as a guide to shape the saddle. That is about as close as you're gonna get w/o being able to physically try the saddle on your horse. (Maybe even closer for many riders . . .)
If you contact Synergist, you will find them very helpful.
Steve Jernigan KG0MB
University of Colorado