You said 8 watt, would be the 6"x8" for a 10 to 20 gallon. If it is the 30 to 40 gallon one it should be 8"x12" and 16 watts.
You are still getting ambient temps, you need to know what the surface temp of the glass is. The pad will get the glass a lot hotter than the air temp just above it. You want to make sure your surface temps are not going to burn you snake.
I dont tend to worry too much about the ambient temps as much as I work about the temps in their hides.