Food for thought: I have gone thru this myself and there have been times where I just went and cut the girl down from 8 weeks to 8 weeks 3 days and went from there with just looking at how much white I had. The result was excellent. There have been other times I went for "the windows to harvest", some was great, some was ok probably would have been better IF I had chopped them down earlier! And doing both kinds of cannabis, it (my opinion) doesn't matter if Sativa or Indica, trichomes rule, you have to decide on the buzz you want.
My last grow I picked at 4 different times 1 at 8 wks, 1 at 8 wks 3 days, 1 at 9 wks and lastly 1 at 10 wks. The results: #1 at 8 wks, #4 at 10 wks, #2 at 8 wks 3 days, #3 at 9 wks, from best to o.k. and these were not the greatest seeds to start with, over 20 years old!