You'd put the probe in the same place that you would if your rhp was a heat lamp, by the uth. The only thing I'd be worried about is your rhp getting too hot to the touch for your snake but I have no experience with rhps. The rule is usually to place the probe where the hottest surface would be to prevent burning your snake and since heat lamps are usually separated from a snake by a screen, your probe doesn't go on the lamp but I don't know about rhps.