On my rbi rack I have a very old Herpstat tstat, and I have it set at about 99 degrees so I can get a 89-90 degree hotspot. But on my homemade 11 tub rack I use a Zoomed rheostat thingy (not sure what its called but it has a temperature set with the temps written on it.) but anyways its set to 95 degrees for a 90 degree hotspot at the top like 5 and like 88 degree hotspot closer to the bottom.