at best these are really just guidlines, not anything set in stone by along shot.. get yourself a 30x jewlers loupe and look at your trichromes with it.. you will notice that as a plant matures the trics start off clear, turn cloudy and then will finish up amber.. you don't want to see any clear heads at all when she is ready to come down.. some people will say half cloudy, and half amber, but it really is up to you and the type of high that you are after to tell you when to chop.. the more cloudy the tric's are, the more of an up high you will get, and the more that they are amber, the more of a couchlock high you will get...
just let the plants talk to you so to speak, and you wont go wrong... so many different growing methods along with so many different growers is why the 8 weeks flowering period is more of a guidline than anything else..
hope this helps you out some.. the old rule of thumb of when you think they are ready, then wait another week is always a good one to follow.. good luck with your harvest..