I am not an expert by any means, but I start 'em at 18/6 till they are ready for flowering room. Some folks start the seedlings at 24/0 for a few weeks, then switch to 18/6 but I'm of the opinion that everyone needs a nap at least once a day, lol. Works great for what and why I grow. (personal meds)
Half the closet is seedlings, other half is juveniles. This way, I don't have to worry about switching timers back and forth. (seperate flower room)