Nuther Opinion...

the second they bust out of the soil and you see the 'hook' form, get them under your flo's ASAP. Place them about 2 - 3 inches from the tops of the seedlings.

Make sure you have a fan blowing full time right down the middle between the lights and the seedling tops AT ALL TIMES.

400W of flo's will cover a small area. I dont think that 18 sq. ft. is nearly small enuff for adequate coverage.

Depends on how many plants you intend to grow.

Initially it will be plently...but as they grow, so will the light demands.