CSK, sprechen sie Englisch?
If you're using a digital thermometer with a probe, I generally lay the probe on the top of the substrate, or slightly bury it to keep it stationary. My snakes do burrow, but most spend the bulk of their time on top of the substrate, which is where you want to measure it.
Although you do want to get the measurement of the bottom glass under the substrate to make sure its not too warm and adjust the UTH if necessary.
I don't think it matters where you put a hygrometer, obviously on the warm side towards the top I'd expect to see lower values. And lower closer to the water bowl you'd see more. Most humidity levels in the average home are just fine for a cornsnake, and if you're seeing bad sheds it can be remedied by adding a moist hide during the blue through shedding phase.
Although from experience, those stick on hygrometers are worthless. You'd be better off buying a good quality digital thermometer/hygrometer combo.
As for the UTH, they're most effective when left on all of the time, that's what they're made for. If you don't have AC during the summer though, you might want to unplug it during the day I suppose. Most UTH's operate at safe temps during average household temps 68-73, at least mine do. I don't use a rheostat or thermostat on mine, either.
Hope that helps a little.
