I will tell you right now, Reptile Prime doesnt keep humidity as well as something bigger like cypress or Reptichips. I can tell this easily in my GTP cage. I have to mist the substrate a lot more. It does hold and release a lot of liquid but it releases it too fast so the stuff dries out pretty quick.
60% is the minimum i would do for a boa or retic. I keep all my boas and retic at 75-85% humidity. You can tell when they are nice and well hydrated. If you feel my retics skin, i swear it is as soft as silk. Feels like you could almost push your finger through it. Same with the boas. They are very soft and supple but not as silky feeling as the retic. And even my BP who keep around the same is soft as butter. When i kept her at 60-65%, he skin was kind of rough and not so elastic if that makes sense.
You dont want condensation dripping off the roof or running down the ceiling though and you dont want a wet cage floor. There is a big difference between relative air humidity and a flat out wet sopping mess of a cage floor lol.
And there is nothing wrong with indented kraft paper. I know a few BCC breeders who swear by that stuff. I also know a few who swear by aspen.