When to veg?

Some of the dozen or so girls are just now legal (18" - 21"). About 5 - 6 weeks old.
Height restriction. Ceilings are under 7'. To the the bottom of the hood is 66". Subtract a foot+ for the smart pot and my height would be 54".
11' x 14' room for bud room, so there aren't horizontal restrictions and 3 1k hps.
I was thinking about transferring in a couple of weeks when they are 24".
I was looking for some input.
I was thinking of chopping the top so it will become bushier.
I feel like waiting for them to get say 30" will just be inefficient.
 
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