You'd need to get a much closer look at the trichs to judge when to harvest. Grab a jewelers loupe or microscope or something to get you in and tight on the trichs. If they are all milky when you harvest, you'll get a head high only... if there is some amber showing, you'll get some relaxation in with that head high.... if there's more amber than milky, you'll be in couchlock territory... but from there on the THC will start to degrade... just depends on what you want from your smoke as to when you harvest.
Good luck...