hahaha ill take that as a compliment! well there actually seems to be a 1-.5 degree difference between the different basking "zones"
Just to be of some assistance here since I use Excavator Clay for many of my Terrariums, you can safely use anything up to around 5mm of Excavator Clay with the mix you have mentioned and it will safely conduct the heat via UTH.
Zoo-Med advise against it as a general rule due to the fact it is common for people to use deep layers when forming their landscapes with this product which would indeed impede the conduction of heat and risk a buildup below resulting in cracked glass etc.
PS: Regarding Air Temps you will in fact want to aim for somewhere between 80-85 F on the warm side of your Terrarium and on the cool side temps can be in the low 70's to room temp, I use multiple's of the same Exo-Terra as you do and generally find a 100 Watt CHE to be the optimal means of achieving this when combined with a Pulse-Proportional Thermostat. Though I would strongly recommend picking up a second Digital Thermometer to monitor this. Correct air temperatures are in my view extremely undervalued in the hobby currently and really do make a noticeable difference to a Geckos activity levels when they have both warmer areas to bask as-well as a comfortable ambient air temperature gradient, otherwise they spend half their time clung to the floor trying to raise their core body temperature.
