Yeah, IR temp guns are where it is at. Ground temp is very important and IR temp guns are very accurate.
I am glad you were able to get a good temp gradient. Your BP will be happy!
When using the temp gun I check multiple spots on each side. I might hit 3 spots on the hot side and 3 on the cool side. Easy since you get an instant reading. That gives me more information. In my BP's tank, it is usually 87-89F under the RHP, but 85-87F where the heat panel isn't directly over. I use heat tape and an RHP together.
The point is if you have a small spot that's 90F and most are 87-88F, for example, that's fine. Additionally, as long as there is ample space for your BP to digest, it is okay if there are areas of the hot side that are a lower temp. A gradient there, if the hot side is big enough, is okay too.
It is always fun to get a new enclosure, for both you and your BP.
Enjoy and keep us posted!