It is likely you will have micro climates in your tank especially for humidity. I like to place hygrometer probes in the warm and cool hides. It is very common to have the arm one lower than the cool.

IR guns are great tools. The basic models are very similar accuracy (+/-2ºF) so the differences are the DS ratio and the batteries it takes. I would recommend one that takes a common battery (9v is common for them I avoid the ones that take things like sr44 I had one years ago that took six sr44 at 7$ a battery... )
DS ratio for royals 1:1 is the best likely they have an optimal measurement distance of around 4 inches. The 6:1 are about 24inches and that seems a bit much for royals.