I rarely have to add water during incubation, but I only take the lid off for a few seconds when I check the eggs every few weeks.
If you do things right and the eggs are strong and fertile you can have 100% hatch rate. (I have always had close to 100% hatch rate of fertile veiled eggs.) The average is likely lower than that though.
Some measure. I never have. I would just check them every few days and mist the container whenever the verm. was dry. Prob once a week. Just don't overdo it or you will get moldy eggs. All my panther eggs hatched in both clutches. I lost two both of those were my neg. They drowned in a planter.