A better way is with the rider sitting the saddle: their seat resting comfortably against the cantle, their feet under them in the traditional straight line from the shoulder - hip - heel position (see FIGURE 1) stand up in the stirrups without using the horn. The rider should have no problem standing up without any assistance if centered. If the seat is to small, the rider will tilt forward (see FIGURE 2). If the seat is to large, the rider will be forced back into the cantle(see FIGURE 3) .
These results are caused by the stirrup leathers. As stirrup leathers pass over the bars of the tree they act as a pivot point. If the center of the rider is forward of the pivot point the riders torso will tilt forward and the feet and legs will move backwards.
If the rider’s center is back of the pivot point the rider’s feet and legs will move forward and the rider’s seat will be forced into the cantle.
Therefore, technically, if the rider is sitting in the saddle with their seat resting comfortably against the cantle and can stand up in the stirrups without assistance the seat size is correct.
The rider needs a seat that is also comfortable. The seat size may be correct but if it is not comfortable for the rider, the result is a negative fit. It the responsibility of the saddle maker to design, build, and shape the seat where the rider is both comfortable and centered.
It has be observed that riders have a tendency to ride a saddle with a larger seat. The reason for this is unclear, but there are indicators. Some of the “rules of thumb” and charts will result in a larger seat than is correct for the rider. Some riders prefer their feet and legs out in front as if they were sitting in a chair. Others feel more secure when pivot effect pushes them back into the cantle and they are braced in the saddle.
The final decision is up to the rider with the understanding that and if the seat size selected is to small or to large for the rider the level of positive saddle fit may be decreased.