Don't worry about the temp halfway up. You need to make sure you have a temp gradient on the gound. 80-90 is good if one end is 80 and the other is 90, but if the whole gorund is just flctuating by that much that isn't right. Don't add a lamp just make sure your UTH is good and has a rheostat on it to control the temp.

What are the dimentions for the tank?