The reason for the probe between the UTH and the glass is just for the thermostat. Thermostat probes need the most accurate reading for what the UTH is putting out, NOT what your inside temp should be. Inside the tank you keep track of temps via digital thermometer or heat gun and adjust your thermostat accordingly. To get an internal temp of 90, the thermostat may need to be set to 95 because like you said, heat is dissipated by the glass. Also there is no right or wrong when it comes to back heat vs belly heat but the UTH is not made to provide ambient temperature regardless of where it's placed. It is to provide a hot spot. Yes it radiates a bit so you will see a slight increase in ambient temps (more so if it's on a tub) but simply put, a heat pad does not put off enough power to heat an entire tanks ambient airspace. If it did, it would be a fire hazard. If you are using a UTH, you will need a secondary heat source (lamp or Che) for ambient temps and make sure you are using a thermostat
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