I wouldn't look at the pistils (the hairs), but at the trichomes (with a magnifying glass).. they turn from clear, to milky and then to amber.
Milky gives more of a head-high, while amber gives more of a body-high..
I personally prefer if about 5 to 10% are amber, while the rest are milky (not clear)..
Since you're using hydro, it's sufficient to change the nutrient solution to water 2 or 3 days prior to harvest.. other mediums have to be "flushed" for longer (2 weeks for soil, about a week for coco), but given you're fully swapping the medium/reservoir, there's no real "flush" of the medium necessary..
If you use something like florakleen or flawless finish, then that can help a bit with removing any salt-buildup from the nutrients..