ambient temps?

Okay I feel a bit foolish asking this but there are no stupid questions right (well, few stupid questions).
Where should I place the probe I hope to pick up ambient temps. I have the cool side probe right near the substrate by the hide, I have the hot side probe inside the hot hide- so where is "ambient" does that mean like in the middle half way down or in the middle down by the substrate or in the middle near the top? I just want to be sure I get a good reading for my thermostat because I may be out of town all day saturday and it is supposed to really heat up around here which will affect my outside temperature and thus my tank temp - so since I will not be around all day to make sure it doesn't get too hot or cold I want to make sure I do this right.

I would take the measurement right about the middle of the tank at whatever level the snake is usually living at. Which is probably substrate level. :)

For an ambient temp I would check above substrate level, where the snake would be when cruising, but I would use the warm side temp to run the thermostat as it's the most essential temp in the enclosure. They'll do a lot better being a couple of degrees too cool for a couple of days than they will if the hot spot is too hot and they get a belly burn.