Not sure, but if I were you I'd try keeping the dome on and keeping the lights as close as you can without overheating the plants. Once they're bigger, you can even have the lights touch the plants so long as they don't burn. I've done this with my golden sage plant. It loves it. Probably not a good idea with new sprouts though.

Personally, I would mist them as needed.

You say you're watering from below? What's the rationale for watering from below? I suppose it would help with keeping the media from being upset. If it's to prevent burning or something, I wouldn't worry about it too much.

I water from below with African violets... and with new sprouts when I use rockwool I guess--just because it's easier.

Lettuce huh? Are you planning to float the plants on Styrofoam when they get bigger? Or are they on a mat to get micro-greens maybe?

Anyway, keeping the lights close should keep the inter-nodal length small (the opposite of "leggy").

Hope this helps^^;