I think there are too many variables to have just one answer.
I think some of it can depend on the types of saddle pads that you use (cotton, wool, felt) and how thick they are, as I think that has some part in heat dissipation or sweat absorption. I think that dry spots can point to one pressure spot, or good contact, or areas of little contact. Likewise, I don’t think that sweat always means contact. For example, when using one of my shaped wool dressage pads, the area under the saddle is drier than the area that shapes the saddle pad. So the parts of the pad that stick out around the saddle actually are “wetter” than the back itself.
Here are some pictures over the years of Mac’s back with sweat patterns from different saddles. The first one was from a western trail saddle with a felt pad. There is a dry spot that I was worried about and I did end up selling this saddle because it bothered me. He didn’t seemed directly bothered by it per se, but I just thought it wasn’t right.
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This next picture is of a saddle I tried after that but didn’t buy. This one it felt like there was more contact on the bars up toward the spine and that the contact wasn’t even across the width of the bars, if that makes sense. This was also a western trail saddle with a felt pad.
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This is a sweat pattern from a treeless saddle. You can see where there is little sweat in the middle is where my seat was, so that would be from pressure, and then the front and the back have more sweat, where there is less pressure. This was a Heather Moffett Vogue dressage saddle with I think a cotton pad.
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I have a couple other pictures, but they were taken into the light so you can’t really see any contrast.
