Get a scope or loup, you can pick up a cheap one from radio shack that does 60-100x magnification.
What's worked for me for 10+ years is to check the number of amber trichomes on the flowers, not the sugar leaves. Count the number of amber trichomes you see in 20 trichomes. Check a couple different spots to get an average number.
One amber trichome in 20 is 5% amber. 2 is 10% and 3 is 15% amber. Less than 5% and the effects of the cannabis don't last as long, which can be as little as 30 minutes. Over 15% and (in my experience) the cannabis begins to be 'sleepy' and narcotic, which is something I'm only very occasionally interested in. (like when I'm sick and want to sleep till I'm better.)
Between 5 and 15% amber, you'll generally find a sweet spot for your particular strain or pheno. You'll notice the terpene and cannabinoid production is fully developed and the flowers are what I'd consider 'ripe.'