How much pressure is on your horse's back?
First determine the weight on your horse's back,
and the surface area on your saddle.

Remember that if the saddle fits badly,
the surface touching the horse may be much smaller;
you may lose 1/4, 1/2, or even 3/4 of the surface area.

For more on saddle fitting, see Saddle
Fitting.
For more on weight bearing and weight distribution, see Weight.

Surface Area
Measure the underside of your saddle to get the length & width.

To get the length, measure from the front of the panel/bar to the
back of the panel. Measure from the surface, where it touches the
horse.

To get the width, measure at the front, back, and at the center.
Make sure you measure from the edge that touches the horse, not the
whole panel. Take the average of the three: for example, if the front
is 5", the center is 3", and the back is 4", then use 4".

Play around with Numbers
Look at some typical saddles: