12/12 from seed works - but only with some strains. I did 12/12 from rooted clone to an early misty and it's almost as big as I would like it if I'd vegged first. I did the same thing with a rooted purple kush clone - it stayed the same size. I'll get like 2 grams off it. I read on the 12/12 from seed club that success varies by strain.
I suggest you look up the 12/12 from seed club and ask them what their experience is with that strain.
Or...you could follow the other suggestion and just veg it for 15 or 16 hours a day.
Good lucj=k
YO