Males should not show white hairs. Hermies most likely will. If you are wondering about a specific plant of yours, post pics. There is no real way to answer how much light is "OK" during dark cycle. It will vary by plant and the conditions by which it is exposed to the light -- for how long, over how many days/nights, other conditions at the same time, etc. Assume any amount of light is undesirable, put a piece of electrical tape or a flap of cardboard to cover the dehumidifier readout.
Hairs and balls ...Females have white hairs, males have balls, hermies have both. Watch for this condition when plants undergo stress, especially when growing from seed. Light ...If you extend your arm straight in front of you and can still see your hand, it is not dark enough. Remember a small light leak (1%) can have a serious impact on your yield(10%).