I'm not the most experienced, but here's what I was taught:
Dirt can be really misleading. Reading a saddle pad is done with a clean pad, and looking for sweat patterns (not dirt or wear), and here's why: if a saddle pinches or puts too much pressure on a certain point, the sweat glands can't function properly, and don't produce sweat. Over time, these pressure points deteriorate muscle and can even cause permanent or really-hard-to-fix damage. The sweat glands not sweating is the key mark of bad fit.
So start with a clean pad, saddle up normally, ride till he's good and sweaty, and look at the sweat marks right away (this is sometimes easier on a darker or colored pad). There should be a pretty clear patch of saddle-shaped sweat. If there's patches of dry where there should be sweat, then you know where the problems are.
Hope this helps
Basically what that pad tells you is that, like every saddle, there is a point billet that hold the saddle very still right over the wither.
Then the back portion of the saddle moves around with the movement of rider and horse's rib cage and make dirt impressions.
You have to get the horse good and sweaty.
or, sometimes what I do is after hosing the horse (before scraping) throw the saddle up there, push down evenly on it and then take it off.
you can see the water marks to know if it sits evenly on the horse. This is useful for detecting bridging.