In my opinion, you should sort his set-up out.. also if you haven't got a thermostat I would regularly check the temps to see if they are ok.
Once you have sorted everything out I think you should leave it a week let him settle in before you feed him.
Once you weigh him, his weight divided by either 0.10 or 0.15.. (for example: 520*0.10/0.15 = 52/78) and that is the rodents weight size you should be feeding your BP.#
Hopefully this makes sense I'm a bit rusty with information.
Just a note on that 10-15% "rule". It only applies to hatchling and juvenile BPs (under ~500g). Most adult ball pythons never need more than one small rat per week (50-80g). Anything more is just a waste of nutrients.