I understand strains have different growth habits,...

2-3x ... you can always HST tall colas.. wait near end of vege growth so you don't have to do it twice... just bend/snap a stem until it is horizontal. It may need support the first day, but after that you'll want to tie it down (loosely) so it doesn't bend back upward. Try to be systematic if oyu need to do this to large portions of canopy.. don't want them lying on top of each other, sos you need to make a pattern that works without crossover of branches... .and still stay in the footprint of light. the only way to know is to have grown the plant before.. preferably a clone, because genetic variance exists even in same-strain seeds. You can't really tell the big stretchers too easily, but more extreme versions of each usually become apparent quickly after flip. The plants that have heavy branching will be stockier/shorter. But some plants just explode after flip and others not as extreme.

As a rough guide, flower them when they get to about 40% of the available height.

From my experience, the plants become 2 or 3 times bigger during the stretch. The stretch begins before the flowering almost as soon as you switch to 12h/12h light schedule (for photoperioed plants, not auto). If you want more details about Height vs weeks, you can have a look at the harvest weeks of my last diary growdiaries.com/diaries/182640-grow-journal-by-smokingtiki. I hope this helps, good luck! 😎