apparently there are strains that produce a lot of amber trichs, but if you aren't growing one of those strains, you want to chop at about 10% amber. while there are exceptions, the general rule is 5 to 10% amber. the darker color indicates that the thc has started to degrade. since trichs are constantly developing over the whole plant, you'll always see a few clears and after a certain point, a few ambers, what you want is 80+% cloudy, 5 to 10% amber, thats the peak of thc content for most strains.
i have a dual 30/60 X jewelers loupe, works awesome, you can get one for 10 bucks on amazon. i look at the top and bottoms of buds, basically ignore sugar leaves, no real science behind that, just what i've always done