Using half the cage is fine, he looks to have plenty of room. Boas are not as easily stressed as BPs, if you really wanted to you could take out the divider and let him have full access to the cage. With some hides and cover he will do just fine.

Regarding your temps, the t-stat should keep the hot spot at a relatively even temperature. You will have to play with the thermostat to get the hot spot temps where you want them. Especially with using a tstat and RHP, and depending where your probe is placed, often you have to set the tstat to a temperature much different than what the actual temperature is on the floor of the cage. Using an IR temp gun makes this process a lot easier. Basically the only thing that matters is that the temp you get with the IR gun, is what you want. It doesnt really matter what the tstat says.

From what I have seen, the tstat probe should be up off the enclosure's floor, so that the snake cant dislodge it, or move it around to cause temp variations. I find most people secure the probe to a wall of the enclosure, under the RHP, and up off the floor. It might take a few tries setting the tstat to the correct desired temps, but once you get it dialed in it should stay regular.