It really depends on the genetic makeup of the plant, but I use about 80% opaque to clear trichomes as a rule of thumb. Also, if I'm new to the genetics, I usually harvest in a couple stages to see where exactly I want to harvest; Experiment! It really is more of an art than anything. You're going to have a combination of different THC, CBN and CBD levels throughout the flowering stage, which change the actually efficacy of the bud.
~ooli~