I see you point I don't truely agree but I get the descision.

The best way to cheaply find a reasonably accurate thermometer is to buy a bunch of cheap ones (3 min and 5+ is better) lay them all out away from drafts and check them against each other. Typically some will read the same these are •likely• close to accurate then you have a base to figure out where what is off. The other option is also cheap but a bit more complicated see if you can find a liquid crystal thermometer that is in one degree or better steps and place this on a surface in the reading zone and then your hydrofarm probe and your other T-stat check them against the LC one. They often (usually) are accurate to one half degree. This will (hopefully) show you correct so you can se if your thermometer is off or the probe or both and how much.