Just wait until there are 95-98% brown pistils and harvest then.
This will give you buds with peak THC levels.
This post once again highlights the problem of basing everything on trichome colour as to when to harvest!
Some strains will never develop amber trichomes and others can have 70%+ amber trichomes after only 4-5 weeks of flowering.
Amber trichomes are largely a function of genetics and not the absolute definitive way of deducing a plants harvest maturity.
Pistil colour/condition, however, is a direct link to plant maturity and it is plant maturity that should dictate harvest timing, and not trichome colour as the only determining factor as to when to harvest.
Once you grow long enough, you will discover that trichome colour varies wildly between strains and can not be relied upon as the sole determining factor as when to harvest.
Hope this helps, Organoman.