I weigh 212 and run 42's at 65lb rear 60 front.
You might try starting at the higher end near what the tire is labeled for and work down to where you start to sense the rolling resistance just start to increase and the ride comfort start to go up.
I have gotten snakebit with a 35mm tire at around 50 lb. (Small tree branch in a dark underpass. Didn't see it in time) Don't go too low.