When I used tanks I placed the overhead heat closer to the UTH than the other end.
Your trying for a heat gradient with one end being warmer than the other so the snake can move from warm end to cool end to regulate it's body temperature.

You need to have a probed thermometer to read those temperatures, anything less won't cut it IMO.
The thermometer itself goes in the cool end to read that temperature and the probe snakes its way over to lay directly on the hot spot to read that temperature.

Your looking for 78-80 cool end, 86-87 warm end and 90 to 92 degrees directly on the hot spot.