How do I properly measure temp?

I am a first time snake owner and I have my new BP's tank set up as most everyone recomends. However I have a question that I have never seen addressed. Where do I measure the temp of the UTH at? Should I measure at the surface of the substrate (coconut fiber)? In the middle (~1" down)? at the glass? I need to know so that I can properly adjust the heat output of the UTH. Ultimatly the temp on the surface is also going to depend on the thickness of the substrate. Any input would be much appreciated. Thanks.

I stick my acurite(Sp) right on top of his substrate. I keep it down by putting the probe wedged under his hide. Anywhere else id be afraid that the temp I'm reading isn't the temp hes getting.