Where is your probe? That info is critical. I have 2 AP cages- 1 with a 40 watt and 1 with an 80- and both are heating perfectly (when hooked up to herpstat) despite being 36 inches long rather than your 48. My probe is under the newspaper on the floor of the enclosure. If my probe was nearer the heat panel, it would read far too high but be perfect for the BP on the ground. You could also use a heat gun or digital thermometer in different areas of the cage to get a feel for what's really going on. I use a jumpstart with ultratherms for my corn cages, and they do go above the temps I set them at but not by much.