Thermostat not equal to the thermometer

[NewBP]

Alright so I have the same issue as the OP. Thermostat says 98 hydro farm says 96. Set to go off at 93. Surface temps though (measured by good quality IR gun) are from 88 to 93 (93 under his hide). Given this info what would you recommend I set the hydrofarm at?

Thanks

Edit: I have the zoo med UTH

Edit2: I have about and 1/2 to 3/4 inch of apsen.