Above substrate being 90 is fine, watch for burrowing at 100 degrees I think that can burn your snake.... For humidity In the pics you have a plastic tub ( I think lol ) if so to lower humidity you can drill more holes so that there is more air flow... Unless you live in a high humidity area, I'm not sure what you'd do then but your right 75 should be fine for when he's shedding.... I'm not much better then a newbie myself so I may be wrong xD

Good luck though xD and have fun!