Distance for gro-lights from seedlings

Hello, Handypeople. I had a heated plant propagator that I got at a yardsale. Base, heating cables, cracked clear dome. My father has taken it in hand. He has built a new "dome" for it, and a stand. Now he wants to put artificial lights over it, but he doesn't know how much room to leave between the surface of the soil and the lights. He's building a light stand now, you see.

He wants to start seeds with it.

Any ideas on how high the lights should be above the soil? I told him about 2 feet probably (I pulled that distance out of the air.) Was I right?

Thank you in advance for responding

BrightStar