For now, put the thermometer probe on top of the substrate. If it's at like 93, then I don't expect the carpet will be that much warmer. If your concerned, put a probe on both.
LOL, yea that probably didn't help any. Put it on the carpet, get a reading, make sure it's constant, then put the probe on the hot spot. The substrate will be a few degrees cooler, within 90-95 is ok.
Oh, forgot to mention. Your tanks are raised from the surface right? They need air space, I believe the UTH's come with little legs to prop the tanks on. Just an after thought, but important to minimize the risk of cracking the glass or fire.