I have back heat so it's a bit different. I tape the Helix probe about a half an inch away from the flexwatt on the back wall of my racks. Doing this gives me the exact temp that I set it for inside the tubs next to the back wall. That being said, I think most people tape the probe directly on the heat flexwatt/uth with belly heat. Rich at RBI suggested the way I do it and temps stay exactly where I want it.
The one tool that I suggest the most is a temp gun. I got mine from Reptilebasics for around $50, but you can get them a bit cheaper. I use it to monitor my room temps and inside the tubs of the racks. It shows the highest temp within a sweep as well as the temp at the last point the laser was pointing at.