Let me clear up some confusion here.
You do a 2 week flush (Normal watering with Ph'd water w/ molasses if soil, DO NOT use molasses with hydro). Now since you're plant is sativa you can start your flush either at around 10-12 weeks or depending on the trichomes. If the trichomes are not amber I wouldn't start a flush, just my personal opinion.
After the 2 week flush you let your plant sit for 3 days (For soil, not too sure about hydro, you might want to ask Mind if there's a pre dry out time if you're hydro), then harvest.
If the trichomes still are not at the percentage you want, do another watering with Ph'd water, wait 3 days and check the trichomes again, until you're at the percentage you want and then harvest.