A question from a fellow grower:
My first grow and my lil babies are 5 weeks in veg. Stage should I start flowering??? Each one is between 7 and 12 in. Only want about 48 in. Is now good time to flower??
I wud wait one more week then turn em. I give mine all the time they need too many people rush and cut corners plz dont do that cos yor yeild n strength will suffer. 
I agree. Patience is the most important quality of growers. 6-8 weeks is a great timeline for a strong veg.
Observation; Flower and finish time can be relative when growing Sativa. The longer you use a long photo period in your veg time; The longer it will take to finish the plant. Veteran growers use a much shorter photo period “indoors” for Sativa.
Sorry. The answer to your question is: You have the right idea. After shortening the light cycle; wait until trichomes are 1/3- 1/2 amber colored , then flush. Happy growing.