This was the best video that I could find on shims!! Appreciated, thanks :)

Thats a great idea! Just bought a saddle for a 4yr old quarter horse and I know he will have trouble with it behind his shoulder and rather than having to buy a saddle that fits him for now, something like this will surely help.

I think Rhonda Martin is very good at demonstrating this common problem, I'm researching for a friends who owns an old Lippizaner cross Italian Draught, & she is a small accurate rider, but the horse is showing his age now, so needs a shim pad..

But, have you checked the new position the scapula moves into when their leg is stretched forward while they are in motion? I found that top and back corner move backwards towards the saddle up to four inches from their original position. So while watching your video I did not see this taken into account. When they are standing at rest the scapula is below the withers, that all changes when they are in motion. I place my shims further back to lift the front two inches of my saddle completely out of the way of the range of motion of the scapula. The "valley" or hollow spot you speak of in this video is the very same place the back top edge of the scapula moves into. Have someone with you who is horse savvy, hold your hand over the top of the scapula, have your friend pick up the horses leg and stretch it forward as if the horse was taking a step, with the leg still forwards find the new scapula position. You will find on many horses the scapula moved backwards up to four inches. I place my shim behind that, not on top of it.