How much of seedlings' stem should be above soil level?
Primarily, my question is that how much of the seedlings' stem (below the growing point; from where all of the leaves appear) should be left above soil level? Or is it dependent on the species of the plant?
I ask because, unfortunately, some of the stems of my seedlings, coreopsis & salvia, are almost totally under the potting mix and the leaves on top seem to be getting bigger and dividing but I have no clue where their stem lies -_- Should I replant them into the same container with their stems above the potting mix? If so, how much of the stem should I leave above the mix level?
Secondly, there are some seedlings whose stems are above the level of the potting mix, but these seedlings, unfortunately, are not developing strong, thick stems and their leaves on top are growing and dividing; so much so that the weight of the leaves is bending the stem to the point that one of the leaf touches the potting mix. Should I replant these a bit deeper?
PS: I am growing all of these seedlings under 16 hours of 6500K, 2 T8 light bulbs indoors. While the leaves are growing and dividing, the stems do not seem to be making progress. Any advice would be appreciated. Thank you.