Probe on Hotside for me 1/2 way between the RHP & floor..
You mentioned having it low as possible because thats where the snake is, well dont forget thats not how Radiant heat works so having it directly under the RHP or having it 1/2 way between panel and floor will give you the same results. The probe is going to draw in the temp that other objects do in your enclosure (including the snake) and that is what gives off heat to get you your air temps.. So keep that RHP probe away from wetness or from being hindered in anyway so that it cant get a correct reading and you'll be good.
If you have the correct size RHP for the size enclosure & a good Tstat then your enclosure Hot Spot will eventually match what you have your Tstat set to (within a couple degrees as no thermometer is the same)
My Tstat is on 91.. My thermometer probe says 87.6 at night when house is 60 and 90.7 during the day when house is 68-70. Laser spot checks are from 88-91 ontop of Hot Hide and substrate.