Get a separate thermometer for your tank, set the heaters for whatever it takes to keep it at 80. Start at 80 and adjust high or low if needed depending on the thermometer reading...
I have three of these, the third set down is 82" the fourth set is 79. I am pretty sure it goes like this 89 - 86 - 82 - 79 and I don't know below this point as I keep my tank at 82. I can't say this is 100% since I have not looked at these heaters in a while they are in my over head. But they have worked great for me.