hiya illa101k2, looking at the Chart(Mh) - 16-18" gives you between 6,446 - 5,093 L (Optimal distance Range) I think plant needs 5000L for decent growth from what I've read..why not just hang a CFL 2-3" above seedlings (if you still got the CFL knocking about) for a week or so and keep lowering the Hps slowly?
Peace