Most strains and different phenotypes of MJ mature differently. As well as their chemical make up being different. With flowers I chop them when the stigmas are receded and the calyxes are swollen almost encompassing the stigmas (btw stigmas are the hair you see, the pistil is a calyx and stigma).
Regarding assessing trichome to asses when to chop. The chemical make up being different in each plant means it's very difficult to apply one rule of thumb relating to trichomes. E.G A 12 week Headband strain I grew would hardly ever have amber trichomes, but got chopped when calyxes are swollen and stigma receded....
Amber trichomes can also be there due to external factors like wind (air flow), the flowers being touched among other things.
Personally I find it a bit difficult to understand how anyone can say, the Trichomes are 25% amber....really? You counted all the trichomes on your plant? (that would be impressive). Perhaps you pick one sugar leaf to do this with, that is not always a reflection of the whole plant ime. Not bashing anyone here, just passing on what I have experienced. In short, I would recommend chopping as I have said, when calyxes are swollen and stigma almost gone.
Here's an example of a flower finishing up. Still has a week or so of flushing...
As you can see, the stigmas are almost being swallowed up....
Peace and happy growing.
DST