I don't measure the humidity. First of all I put down a bed of moist spagnum moss onto a large square plastic lid. Then I nestle the eggs into the moss, allowing them to touch each other. Then I top this with another light layer of moss. Every time I check the temp, I lay my hand on the moss. If its drying out, I spritz it with room temp water.
I learned how to hatch eggs by trial and error with box turtle eggs. Not moist enough and the eggs would collapse in on themselves. 10 years ago I hatched out 18 Manouria eggs, and I've been successful at hatching yellowfoots, box turtles, leopards, sulcatas (years ago), mud turtles, etc. My problem hasn't been in the hatching...its been in the nesting. Next year I'm going to try cutting down branches of the trees I have here...mullberry, Calif. pepper, Chinese elm... and cutting them up into small pieces, leaves and all, and see if that would be a good nesting medium. I don't like having the sticks included, but in order to provide her with enough leaves to make a mound, I would have to cut the whole tree down!! She uses A LOT!
Oh, I'm sorry! I didn't take it that way at all. I'm just not very scientific with my turtles and tortoises. I just go by the "seat of my pants." and use common sense.