The temperature I or other set there t-stat at really does not matter, what matter is the temp YOU need to set YOURS at.
There are a lot of thing that come into play such as ambient room temp, whether the rack is enclosed or not, tub used, substrate used, temps YOU want to achieve etc.
You need to decide the temp you want to achieve and based on that play around with your t-stat to see what it needs to be set at to achieve that temp.
I have t-stat set as high as 98 to achieve a 88 hot spot just like I have t-stat that are set at 85 and to achieve a hot spot of 88, depends on the rack, room etc.