Seedling does not require too much water, if the water run off in 15l pots and there are no roots yet to suck up the water in excess it will take so much time to dry and you are not going to give her water (maybe with different mute schedule) for some weeks. I would just give her some water near the seedling where it could have put some new roots and add some more every some days..
Don't worry about salt build up. Never had problems like that, just don't exaggerate!
Good luck