OK...here you go again everyone...sorry if this gets redundant but looking for some advice.

I have a female BP, about 116g and 18 inches long. I have had her about 3 weeks now. I have her in a 10gal tank with a screened top which I have covered with plastic wrap to help hold humidity in. I have 2 hides for her, one at each end, her water is about half on the UTH and half off to help not only warm the water but to help with humidity as well. Temps are very unpredictable and only within a couple degrees of each other from the cool end and warm end. I have one of my temp probes under the hide on the warm end to give me belly temp in the hide which is over the UTH and under the heat lamp and temp is 87.3*F and the cool side ambeint temp is 85.6*F. I am having trouble getting a basking spot to the 90*F range. It seems that with a tank this small the temps are going to be very difficult to maintain. I drop the wattage of my heat lamp and the temps get too cool. Any advice on how to better regulate the temps to get to the 80*-85* range and the 90* basking without overheating the tank? How much "give" is there in the temp range without being too hot or too cold?