I hang my probe about 3 inches to the side of the RHP about 2 inches from the aspen, then set the Herpstat to about 5 degrees lower than what I want the hot spot to be. For my cage its set to 87 and gives me a hotspot under the RHP of around 92. I use a temp gun to fine tune it and the reason I hang it 2 inches from the floor is so when my red tail boa goes and curls up there, I wont get a false reading with her smothering it.
And like Norwegn113 said, my set up is for a terrestrial snake. If its for something like a GTP or GTB, then you want to mount the probe to the panel or hang it right next to the panel about 1 inch down. Basically, you want to set the probe at the height the snake is going to be. I also have a flat rock right under my RHP to also radiate heat from as RHPs heat the air primarily and anything directly under them. They don't function like a UTH.