How tall you let clones get before flowering is probably strain-dependent. I did a mini-SOG experiment 6 or 7 weeks ago, flowered 6 AK-48 clones about 5 days after I put them into 1/2-gallon pots. They are quite large now, at least 3 feet tall. So, I did a second experiment about a month ago, also AK-48, 24 clones, 1/2-gallon pots, but flowered them 2 days after potting them, and they're still getting quite large. I guess with AK-48, you should put them in their pots and flower them *immediately*, the same day, else they will get quite large. I think I have enough vertical space, but sheesh, I didn't expect little 3-4" tall clones to turn into these 3' tall monsters...and they still have a month to go!

I'll second Lumatek ballasts via bghydro, good experiences with both.