How hot is too hot for the surface (meaning bottom of the enclosure) temp.?

Also, for some reason, I'm now having a hell of a time getting the temps right in both enclosures. Sophie is in a ten gallon glass tank (this will change to a 50qt. rubbermaid this weekend), Mark's BP is in a 50 qt. rubbermaid (we will be buying a 90 qt. this weekend). I can't get the temps. in either to go over 80/85 degrees on the hot side (low sides stay between 75 and 80, so those need to be higher as well). I'm using a heating pad in both (the $10 one from Walmart)....HELP!!