My SoG's used to have a 250hps in an air-cooled hood, with the bulb itself about 30" above the tubs, in a cab. Chamber was configured for 3 stages, so light needed to remain at that height. New stage in the middle, with 2nd stage to the left, and 3rd stage flush to the right side of the chamber. Clones were scheduled to insert in the chamber every 3 weeks, so took them 10 days before. Clones were 3" when inserted in the chamber. Indicas finished off 10" - 14". SweetSkunk (SweetPinkGrapefruit x BigSkunk).. 22" - 24".
2 possibilities though.. 3" clones are stretching up to the light 30" above, or, stretching due to just hps. The SweetSkunk was a monster though.. so may be genetic.. indica/sativa.
My next cab will be a bit different.. going to set up like a stadium (step),, but instead of vertical light, it'll be horizontal (LED panel), and the 1st stage will be at the top edges, which then gets dropped down 5-6" for the second stage 3 weeks later (or whatever duration strain needs), followed by another drop down for the 3rd stage/flush in the middle of the ' \_/ ' in the final 3 weeks (or as needed).
I'm "hoping" this will be better than my last set-up. Babies won't have to stretch up to the light so much, and the overall canopy will be pretty even. A few books in the past mentioned putting tall plants in the middle, with smaller plants on shelves to the outside perimeter. So that's my goal.