Is there anyone you can borrow one from to try before you buy?
Otherwise, I usually advise to fold up a towel so it's about 1.5 inches thick and put it over the withers, on top of your saddlecloth, but behind the shoulder (not pulled forward so it sits on the back of the shoulderblade). Then check if the level is better before you ride on it - the pommel should sit slightly lower than the cantle.
If it seems to work okay you can fiddle with the folds - you sometimes end up with a gap at the base of the withers using this method but if so that should be easy to fill by extending the towel back further at half its thickness, or whatever you need.
With this type of problem you often need to experiment a little to find out what works best. And it's handy to have someone else check how tight the saddle is at the front (behind the tree points) and mid-panel while you're on board.
Hope that works