Measuring my temps?

So I got my first ball python at Daytona. I'm just a little confused on where I should be measuring my temps on the warm side. I usually put the temp probe right on the glass for my other snakes but they are all burrowers. So where should I be measuring the temps, on the glass or on top of the substrate. If I do the glass then the surface is too hot. And of course if I measure at the substrate surface, then the glass is around 100+!!! So I'm completely confused on what to do. I want to offer him some food but I'm waiting until I'm sure I have this temp thing under control. On a side note, what size mouse would be good for him? I currently have fuzzies and sm mice but could go to the store to get him something else. I attached a pic of him for reference. Thanks so much for dealing with my spasticness lol. He's just so cute and I want him to be as healthy as possible. Thanks!

http://i872.photobucket.com/albums/a...f/100_2984.jpg
http://i872.photobucket.com/albums/a...f/100_2985.jpg

do you have a rheostat/ thermostat for your UTH? i put my temp probe in the middle on the back wall of my bps cage for ambient air temp. i have a rheostat for my UTH and i measure it every morning and i measure inside the hide over the UTH and make sure everything is up to par. umm my bp is about 17 inches and i feed her small adult mice and she is fine so you can try adult mice. the mouse should be as big as the widest part of your snakes body.

I have a rheostat right now but I don't like it. Its not as stable as I'd like so I have another thermostat on order which should be here in a few days. I have an acurite in the tank so the main piece shows me my ambient temps which are about 80. I just don't want the hot spot to be too hot and I'm not sure where I should measure those temps. The difference between the glass bottom temps and the substrate surface temps are pretty big. So I'm confused by that part. :(

I measure mine under the substract that way I know the temps where he can get to. Mine likes to move all the substract to the sides in his hide so he is usually down on the glass so that works for me and I know he isn't going to get burned.

EDIT!
BTW that is a beautiful snake you have!!!