Heating outdoor enclosure with heat pad?

I have three sulcata tortoises all are about 30 to 40 pounds. I have the rubbermaid tortoises house shown in Sulcata Station the URL is http://www.sulcata-station.org/ and I use a Stanfield heat pad and two spot lights to heat the house. I have a T-stat that only lets the heat pad and lights come on when the outdoor temp goes below 50`F. The heat pad I have now is a 2x3 and I am ordering a 3x4 because the tortoises out grew the old one. Not all three can fit on the 2x3.

The question I have is this setup OK or do I need to put a rheostat on the Heat pad. I literature from Stanfield says that the heat pad will only get " about 30 to 35`F above the ambient surrounding air temperature". So when it comes on at 50`F the heat pad will only get about 85`to 90`F. And , please remember that the Rubbermaid house is not even close to being air tight.

Thank You for any and all replies.

Fatman

I don't buy sulcatas to not deal with these housing issues. If i lived in florida or socal i would have aldabras for sure.Anyway.....
If i were you, i would probably just leave the heating pad permanently on when the temps are below 50, and connect to the stat any other time.