Heya!
If you judge by the pistils usually 70-90% ambered is a good time to harvest. The best way is to use a small microscope or jewelers loupe to have a glance at the trichromes. Most people harvest when all the trichromes are milky to 30% amber. If you harvest with more milky trichs it will produce more of a head high effect, where as more amber trichromes will give you the couch lock effect.
I would suggest looking at the trichs if you can a little closer, I have had plants ready to harvest with lots of white pistils left still and some where they were all brown.
Cheers and good luck!