Where are you measuring that 85 degree hot spot? Inside the tub? On top of the substrate? If it's set at 92.5 and it's only giving you 85, then you need to set it higher. Unless your thermostat probe is inside the tub (not recommended unless you have a dedicated tub just for the stat probe) then you're going to need to set it higher than 92 to get a proper reading inside the tub.