18/6 is the best, no more - and all plants need sleep so definitely not 24/0!
Remove the cover too, your seedlings need good air exchange to grow properly and air movement to strengthen the stems.
They do not need extra high humidity, this is an outdated concept from the 1960s when no one knew much about cannabis and thought it came from jungles or rain forests, which it certainly does not.
If you are planning to put these outside, you should be trying to match the photoperiod indoors with what you expect the dawn to dusk times will be when you move them outside.
If you have them at 20 hours (which is bad anyhow) and move them outdoors and they are only getting 13 hours of light, this could trigger flowering or other freaky outcomes, like hermaphroditism.
Normally, if the time between dawn and dusk is say 13 hours, you can add on another 90 minutes as it starts getting light before sun up, just like there is still a little light after sunset.
Therefore your outdoor photoperiod would be 14 and a half hours, so an indoor photoperiod of 15 hours would be the best match for your indoor (and soon to be outdoor) seedlings. These thoughts should guide your indoor lighting cycle, however if there is 14 hours between dawn and dusk at your planned transplanting date, then 16/8 would be safe indoors.
At the moment, your plants are quite stretched, which can be caused by high humidity, but in this case I would say you could lower your light a few inches to prevent further stretching, along with the removal of the humidity dome lid.
High humidity can also cause all sorts of fungal problems which can be fatal to seedlings.
I would also suggest transplanting them into bigger pots in the next few days, those tiny cups only have enough room for a few days of root development, after any more than a week in those tiny cups, it could induce stunting.
They really are only sprouting pots, not seedling pots.
Not being familiar with your light, if it is a flouro light, half the distance you have it now should be safe.
Hope this helps, Organoman.