First, you want this thermostat.
https://www.spyderrobotics.com/index...products_id=76
Secondly, the probe goes between the UTH and the bottom of the tank.
The probe for the CHE should go in the tank about halfway down, but close to the substrate. If it's a screen top, and does not have a cutout for a probe wire, you can put between screen and CHE. I don't recommend this, but have done in a pinch in the past as to not upset the integrity of a screen top. There is always a big difference between probe and floor temp in this instance. Huge difference.
Currently, and temporarily, I am running that for Toref, my monitor. It read 100F at the probe, but is 85F at the ground 12 inches below.
The temps at the probe when not directly touching the heating element, like a UTH, will have a discrepancy between its reading and the actual ground temp.
That's where the temp gun comes in. For example, the temp gun may read 84F on the ground, but the thermostat say 88F. That's fine.