The best place to measure simply for information would be on the bare glass of the tank over the heat mat, but if you're talking for a thermostat probe then it would be best directly on the heat mat, outside the tank.
1.0 Normal - Maynard
1.0 POG - Victor
0.1 YB - Diana
Okay, i put the probe on the bottom of the tub. We're using a 23 foot heat wire mounted on the outside of the tub in a coil along the bottom. We had a spare dog crate door thats folded in half and elevates the tub about a quarter of an inch off a tile floor to keep it from over heating. So, should I put my thermometer on the bare tub and measure the temp or on top of the substrate. It's a circle gage thermometer so it seems to heat up quite a bit under it and I don't think the temp reading is accurate. Also, what do most of you keep the humidity at in your tubs?