OK, some of my snakes prefer the cool end and some will actually push the supstrate away from the hot end basking area to get a little warmer. I think they are just like people and some just prefer differnt areas.

Temps 75 to 90
humidity best at 60 to 70

a hardware store will sell a rheostat light switch for 10 to 15. I got one at Home Depot for 10.49 and it plugs right into the wall. Just remember that when its light is brightest it is off ( so you can see it in the dark). Play around with the setting until the basking area is around 90. Using a simple thermoater is fine just make sure it stays on the area where the UTH is at and that is where you want it to stay at 90.

If foil is not working for about 10 you can get a piece of plexy glass ( hard plastic that will fit over your tank. Just drill about 10 quarter inch holes at one end, preferable the side fathest from the UTH side.