Thermometer and Temp question

Yet another temp based question so I'll apologise in advance!

Right, I measure my temps with a good in/out thermometer and both my UTH's run through reliable thermostats. I measure the hot side of the tank with the probe of the thermometer (which is placed on top of the substrate inside of the hide) and it stays around 94F. The cool side of the tank is measured by the actual thermometer unit itself, and reads around 81F. I noticed today however that the sensor on the thermometer unit is actually about 1cm above the substrate. When I moved the probe to the cool side and placed it on the substrate it read 89F!!! I know the obvious thing to do would be to turn down the cool-side thermostat so that the thermometer reads somewhere in the mid 70s (substrate therefore actually being in the low 80s) but surely my abient temps are then too low?!
Sorry if that was difficult to make sense of but I hope someone understands?!
Cheers!

I had the same problem with ambient temps being too low, so I had to use a low watt red light at night to keep the temps in range. I would definitely try to get the cool hide between 82-84*. If you have A/C running then you can shut the vents (I had to do that too), it helps a lot for ambient temps.