Have a look at the BP care sheet, if you haven't already. We have the 20 gal critter tank, too. We ended up buying more supplies so we have this:
1. two identical hides - one on the cool side and one on the hot side
2. Aspen shavings - started using this instead of the fir bark, recommended by someone, and to make the substrate deeper to make sure he doesn't have any chance of getting a burn on the glass bottom due to the UTH
3. two little battery operate gauges that have both temp and humidity - one for each side
4. a rheostat to control both the CHE (ceramic heat emitter, in a lamp on top of the tank, but not touching the screen) and the UTH (under tank heater) so that it clicks on when needed, shuts down when too hot
5. a large ceramic water bowl to increase humidity (more surface area)
6. a bottle mister - we mist often now that humidity is dropping due to the arrival of autumn
7. a fake plant to add color and hiding spot
8. a fake branch to climb over the water bowl or bask
9. an extra hide in the water bowl in case he feels modest about his bath
10. a heat/humidity barrier on the top of the screen except for where the CHE lamp is and a hole on the cool side to allow for air flow

Sounds like a lot, but Mr. Snake is very happy. And we no longer struggle with temperatures; and humidity is less of an issue. Humidity is very important and especially so around shed time.

The white spot on his head may be either a sign of it being too dry (just wet your hand and pet him and see if it helps) or a sign that he may go into shed soon. If he really ate recently, try not to handle him too much until it's been about 48 hours or so to allow him to digest. The warmer temps of about 90 degrees are especially important during digestion, as well.

Good luck! And don't worry, it's not as hard as it sounds. It's a labor of love!!!