Well it all depends on what kind of stone you are after.

To get an up, energy high, you will want to harvest when the trichs are 50% cloudy, 50% clear.

To get a couchlock, narcotic high, you want to harvest when the trichs are 50% cloudy, 50% amber.

I harvest when my trichs are about 85% cloudy, 15% amber. That gives me a mix of the 2. This all an approximation, not exact numbers. Hope this helps, peace