Running 12 myself under each 600 nanolux dei used to grow my roots 8" to 12" long in the aero cloner before transplanting into 15 litre pots. i used to fill my pots with coco or organic soil and push my hand and arm almost to the bottom of the pots to make a channel to drop the roots into and then gently push the soil to cover the roots.
i used to pack 9 pots to 16 pots under one 600 watt light and veg for two weeks befor flipping to 12/12 and i did well.
i grow in flood tables these days and the odd nft grow.