Try contacting Collegiate directly and ask them.
The saddle should have at least 1 plate on it if not 2...numbers on that plate should help identify what is "in" that saddle.
Collegiate can also tell and explain to you how to measure their saddle exactly to give you the best guesstimate.
Personally, I use "tree-width" as a so-so rule... what I do always go by is how well the saddle fits the horse first, then the rider.
Without a comfortable fit on the horse the rider is toast in achieving the best out of their mount and enjoyment for both of them.
Best of luck.