depth for planting a leggy seedling

I just germinated my first seed everything is going good the only problem is she I getting too tall to fast. I have here an inch from 2-40 florescent bulbs
Her roots are coming through the rock wool an I am getting ready to put her in hydro
How deep can I put the stem into the hydroton
Her first leaves are 3" from the top of the rock wool
Thanks for the help

All the way up to the leaves.

Thanks