I put my english saddle on Ri for the first time today. He didn't really seem to mind. I put it on and let him wander around with it for a while and the mounted. It was really wet so we only walk trotted but he never flicked an ear or seemed uncomfortable but I wanted to make sure the saddle fits right. (It's my HDR)... it's not the quarter horse one they came out w/ this year but it's the one they were selling for QH's last year...
First things first - you must ascertain exactly where on the horse's body the saddle should sit. Horses having a lot of Thoroughbred blood are rarely difficult to decide where the saddle should sit - they have a pronounced indentation right behind the shoulder blade and if you try to place the saddle anywhere else it will quickly find its way to this "sweet spot." Unfortunately, many Warmbloods as well as other breeds such as Arabians and Morgans, frequently lack this clearly defined area. The saddle should be placed immediately behind the horse's scapula - not on top of it. To determine where the scapula is located, have someone lead your horse for you while you walk beside him with your hand on the shoulder blade. As he moves it will rotate about its axis and allow you to visualize its location. Place your saddle just behind the scapula and girth it sufficiently to hold it securely in place. Keeping the rule of balance uppermost in your mind, stand to the side of your horse and draw an imaginary line through the center of the stirrup bar, perpendicular to the ground. Draw a second imaginary line through the deepest part of the seat, again perpendicular to the ground. As mentioned before, the deepest part of the seat should be approximately 6-7" back from the center of the stirrup bar. The illustration at left shows a saddle that is too narrow for the horse and consequently sits with the deepest part of the seat too far back from the stirrup bar. The last illustration of this series shows a saddle that is too wide and has the deepest part of the seat too close to the stirrup bar. The former will place your legs too far to the front while the latter will tend to tip you onto your crotch. Width
Once you've ascertained that the balance is correct, check the width of the tree by stepping to the front of the saddle and observing the fit along the horse's barrel. The "welt" of the saddle (the round piece of leather that runs across the pommel and down either side) will usually correspond fairly accurately to the shape of the tree of the saddle. If you lay a riding whip along the horse's body, starting at the top of his withers and right in front of the saddle, you can compare the actual angle of his body to that of the tree of the saddle. If the two angles are very nearly identical, the width is correct. If the two lines converge over the top of the horse's back, the saddle is too wide, while it's too narrow if the lines diverge. A word of caution here: It is the lesser evil if the saddle is slightly (and I do mean slightly) too narrow than if it is slightly too wide. A very mildly narrow saddle is an inconvenience to the rider because it will tend to cause his balance to be a bit too far back but nonetheless results in an even pressure along the panels of the saddle and no discomfort to the horse. A slightly wide saddle will increase the pressure in the area of the bars of the tree and result in discomfort on the part of the horse, who will usually react by tightening his shoulders and shortening the reach of the forelegs.
You can't tell with a pad. :) Take the pad off and lay the saddle directly onto his back. Slide your hand down the front of the saddle, if it slides easily without much stickiness, that's good. Feel if it pinches his shoulders or withers. Put a hand on the pommel, and one on the cantle. See if it can easily be rocked from front to back. If so, not good. If not much movement, that's good. Lift the flap up, and see if you can see air under the saddle right above his back. If so, that's called bridging. That's bad. :) That puts two uncomfortable pressure points one in the front and one in the back due to the saddle not laying level on the back. Stand back and see if the deepest part of the saddle sits level. If the cantle sits lower, not a good fit. You can correct with a lift so long as it doesnt put unwanted pressure in the front.
If the pommel sits lower, it's probably too wide.
Hope that helps!