In my first system I grew lettuces, cabbages and peppers in 6 inches of grow bed with plenty of success... with that said, most people target around 12" and I now subscribe to deeper is better.. But it's not going to stop you from sprouting and growing lettuce. Some plants do better with deeper roots and some plants just fall over in shallow grow beds.

It looks like you have some extra depth available to you in your grow bed. If your stand can handle the weight, why not use it?

brian