m0useanswered grow question 3 years ago The manufactures guidelines are for when to harvest are more like suggestions.
You will need to pay attention to your plants own needs. It may be around there but not guaranteed at 9 weeks.
Best to look at trichomes as your best indicator when to harvest.
As for flushing, Its a highly debated topic. If you want to flush, do it a week or two before you plan on harvesting based on trichome maturity and your setup.
I myself do not flush.
Best of Luck and Happy Growing