I grow various lettuces (mostly mesclun) under my lights in Winter. I grow them in three 4"x20" window boxes. I use standard potting soil and they grow great.

Just try using standard potting soil! Forget the fancy soil and fertilzers.

Thinking about this more... I keep the mesclun greens growing right up to and into the fluorescent grow-light/daylight bulbs. It doesn't seem to bother them at all.

And I snip all my greens about an inch high. They re-grow. And if a few die back, just put in a few more seeds.