Junji98, I would lay your probe on the substrate so that way you get a more accurate raeding. It could be 88 hanbging off the glass but the actual surface temp is going to be higher then that as the heat disappates as it travels up.
Find out what that actual substrate temp is.
Like i said before if you cant or dont want to get a thermostat for your UTH, you can go to home depot and they have Lamp Dimmer switches that are all raedy to use for around 10 bucks. (Usually hanging on a clip strip or somehting by the lamp and cieling fan section or you can find them in the section with all the electrical switches) They work great and I know of people that use them instead of t-stats. ALthough IMO a t-stat is much safer in the way of, if your room temperature increases or decreases the temp of your substrate will also. Probablly not enough to be concerned with unless your heater gets stuck on high and your room heats up or cools down dramaticlly. Temp flucuations in your ambiant room temps is when a t-stat can prove to be invaluable! :)
Good luck!