Typically, IIRC, ideal humidity for camera lenses is 45% to 55%. Too much humidity leads to fungus in lenses. Too little humidity leads to drying of lubricants in the lenses and possibly cracking of rubber, if any
That depends. Just leave it for a day or two and look at the humidity. If at desired level, then maybe you won't need to adjust much. Or else you will need to adjust slightly and see the change. That is for my digi cabi. I believe some others are different.