I use and 40W RHP mounted in the center of the tub (30" by 15", 14" tall) to set the standard mid-grade heat (81-83F) and a UTH on one side to augment the heat in that part to about 91F, which leaves the other side of the tank with only the perimeter of the RHP heated "footprint" to drop to about 77-79F at the furthest edge. Pretty happy with that gradient, and i see my boa utilizing all parts of the tub throughout the day.
I also have a climbing branch that spans from the cool side to directly under the RHP, he will occasionally climb up there for a high-heat bask, but never for very long.

Note: my RHP is on a dimmer for control and a T-Stat for safety. My temps are achieved with the dimmer at about 80% power, the ambient temp in the room is about 68 to 70F.