I have both my tank and tub set up to a dimmer similar to that one pictured. The room they are in stays pretty constant temperature wise, so I do not have to do too much adjusting. Usually, I check in the morning and evening and adjust then. When I first bought them and wanted to make sure the mats would never spike about 95 degrees, I waited until the warmest part of the day, checked the temp and adjusted from there. So now I know that I am in the ball park, so there is not too much to worry about.

A very affordable solution to not having a thermostat.