On those readings you had were you aiming the temp gun at the substrate or the actuall glass of the floor. You should get the glass surface to the desired temps and then put the substrate on that. They will dig down to the glass if they want to be warmer. If you get the substrate surface to 90 degrees it is likely your glass is way to hot and they can get burnt. It would also not be at all uncommon if you are shooting the substrate for the temps to be varying like you described. My glass surface varies a couple degrees if I aim at different spots of the UTH. I just make sure the warmest reading I get is 89-90,then put the substrate back over it.