Boy you've done a wonderful job with this grow! It's a bit hard to tell how much longer you've got but I think your instincts are good... certainly no LESS than 4 weeks... I'd keep flexible, though, and wait until you're a little closer before making the decision on when to harvest. If you can, get yourself a jewelers loupe so you can get in tight on the trichs... if you harvest when they are all milky, the high will be an energetic one with little-to-no relaxation... if you then wait for them to become 10-20% amber, you'll have a nice, balanced high - but if you wait until they are mostly amber, you'll get couchlock and the THC will start to degrade quickly.
Again - magnificent job! You should be very proud!
Good luck!