How hot is your actual flexwatt? I wouldn't feel comfortable running flexwatt that exceeds 105 degrees. It's not really meant to go hotter than that.

The cold side sounds fine. Your hot side should be measured at the bottom of the tub under the substrate. It shouldn't exceed 95 degrees. That way if your snakes decide to burrow down, they won't burn themselves.

If you can't heat the house, can you heat the room the rack is in? I recommend a stand alone heater in your snake room. 72 is a little low. 75 would be the lowest room temp I keep snakes in.