Sounds good as long as you get the RHP to heat the ambient air as UTHs don't do that. I personally don't use UTH in my HDPE cage, I just use a RHP. Even in my 2 glass tanks I just use a CHE set up on a Herpstat and it works fine. I would guess you would probably need to set the T-stat to something around 110F or so to heat enough through the PVC but also you are using 1/2" of substrate which I believe is a bit thick. You want something like 1/4" so the heat can actually get through. That is why I don't use UTH as I usually run about 1-2" of aspen as my BP likes to dig in it and make little tunnels to her hides. A RHP is a great way to go, one of the best imo. They last forever, wont burn your snake, and heat the ambient which is the most important heat for a snake. I personally wouldn't stick a temp probe into substrate or even on the floor since if the snake pisses on it or spills its water dish on it, your T-stat will kick in and think the cage is cool and heat it up. Plus all that water can also destroy the probe. I guess for a BP it isn't too bad but for my BCI, if she pisses on it, it would be destroyed since she pisses a tsunami. The only way really to monitor the bottom in like you did, probe between the UTH and the bottom of the cage and then just temp gun the inside and raise the T-stat until you get the temp you want inside.