It looks like you have a while to go before you can allow the humidity to decrease much. I have never grown Lithops but my operating principle with most succulents that make very small seeds is to let them bulk up for a while (weeks to months) under plastic, so that they have a decent sized reservoir to draw upon when the plastic is removed and the soil starts to dry out. Maybe the size of a peppercorn or more.
You can safely assume that when you leave the plastic lid on, no water is escaping the container. There should be no need for watering then, really. And actually it's probably better just to let the little guys grow up before you change the environment in any significant way. Provide strong light so they can do their thing. When you do lift the lid, try to make the transition gradual so there's not too much stress. With holes in the bottom of your container (and an exit below for the water) you can mist pretty regularly at that point, and any excess will go away.
You can start to fertilize while they are under plastic or wait until the top is off. No rush, the seed packs a pretty good starter kit of nutrients. Use a low dosage (usually a quarter the recommended dose or less). I like to bottom water (place the container in a tray full of water, give it half an hour to soak some up, then remove it to allow all the water to drip out afterwards) so that I don't upset tiny sensitive little plants by bombarding them from above. Whatever works for you. I'm sure Lithops require some special care down the road but this should at least get you started.
