^^Zwyatt has it right on the mark.
Like he said, what's the point of measuring the substrate if the glass is going to be much hotter (or something to that effect).
The key is to attain a 'happy medium' of your temperature gradient.
You have the cool side, right. Then you need the hot side. Remember, (you probably already know, but..) snakes cannot regulate their heat with out your help. And, at the same time are not the brightest when it comes to moving away from heat that is too hot. A too high temperature leads to the obvious of burning the snake, but also can lead to other problems (respiratory problems for example).
Anyways, you will need to experiment a little to find a 'happy medium'. The range is something like ~82-90f or the hot side. If the glass is just below 90, the top of the substrate will probably be around 82. This all depends on the type of heat source, the thickness of the aspen layer, and the particle size of the aspen. The smaller the particle size, like sani ships verse the more common shavings is going to hold more heat and be hotter.
To try to clarify my above post, I'll give it another try:
1. The under tank heater heats up(highest temp)
2. The glass heats up(pretty close if not the same as the highest temp--some heat dissapates across the entire glass, some heat transfers to the aspen, some heat transfers to the air)
3. The aspen heats up (lower temp then the bottom glass---the aspen closer to the glass will be hotter than the aspen farther away from the glass)
4. The air above the uth and aspen heats up (lowest temp)
As this transfer of heat occurs, heat is dissipated and essentially lost (not really but for the purpose of this discussion it is). The key point to remember is the highest temperature, which the snake could potentially come into contact with (which it will, if you are using aspen, because cornsnakes love to burrow and make tunnels through the aspen) should be no more than 90----this is on the glass below the substrate.
Hope this helps a little bit more.
