Usually the plant stretch the first 3 week when flip to 12/12, in these 3 weeks you can see them grow 1 inch per day and even more. In you're case at day 28 you should not have any problem. Checking the FAQ's you can see the chart : Flourescents: proximimty, 400w HPS: 1 foot, 600w HPS: 1.5 foot, 1000w HPS: 2 foot. So any lamps less than 400w 6 inch should be perfect.