How do you check a saddle against a withers tracing?

Can anyone describe how to check a saddle against a withers tracing?

I used a flexible curve gently molded to my horse’s back at his withers pocket where the tree points would fall and then transferred it to cardboard and cut it out. At the tack shop I placed it inside saddles at the tree point location. I found one that the angle of the panel’s and the angle of the tracing were almost perfectly aligned and that allowed for 3 fingers to be inserted above the tracing at the pommel and two on each side. Took the saddle home for a trial and when it was placed on the horse it came to rest almost on top of the withers. I couldn’t get more than 1 finger around the withers. Obviously not good, but my question is, what did I do wrong and how do I do it better next time. Horse is a 17 2 hand tb with high and long withers. He was a lesson horse prior to us buying last week and you can tell that his previous saddle didn’t fit great and my current saddle doesn’t either. Got to find a solution to make him comfortable. Obviously getting a saddle fitter out would be ideal but they are kind of hard to come by near me.

There’s more than just assessing the front opening. We also assess the lateral shape of the low point of the back and the longitudinal shape. The saddle might be the right width for the shoulders but if the panel isn’t the right depth to support the width then the saddl can fall down on the withers. This can happen on horses that are narrower in front, dippy, downhill, have wither pockets and more. All of these can be on hoop tree or Thoroughbred types.

Do multiple tracings and label them according to where on the back you took them. Do one at the withers/behind the shoulder where the tree points will go. Do one mid-back. Do one behind where the cantle will go. If you want to get really fancy about it, also do one that follows the curvature of the spine. Then you can cut them all out on a piece of cardboard and when you get to the tack shop lay the saddle out upside down on a towel or blanket and place the cutouts accordingly (including the spine curve - that one is important, too).

Of course remember to mark them as to which is which! For the spine curve one, you can cut it out thinner so that you can lay it in the gullet first and then place the other ones in their respective places.