400w hps 2'x4'x5' grow tent insight

I currently have three clones growing under CFLs. My question is, if I am running a 400 w hps in there with 120 cfm intake and 6 in line 240 cfm exhaust, at what height would you switch to flower so you don't run out of the room? I would use a 3 g or 5 g soil grow. Also, I plan on LST all of them and topping.

Height will vary drastically depending on strains. Typically the plants would double in height, but some strains can even triple. Research on each strain would be needed to come to an actual conclusion.

Honestly man I ran a 2 x 4 x 5 tent I vegged my gorilla glue plants out in 1 gallon buckets for 1 month i also lst'd them but I did not top next I flipped too flower and I had no issues with height but I never transplanted into bigger pots so if your using 3 or 5 gallons I dunno.....
The medium i was using was rockwool grow cubes and hydroton clay pebbles.