If you want cerebral, harvest now/soon or when there are 95-98% brown pistils and just accept whatever the trichomes are.
Another 2 weeks and the pistils will be all brown, meaning a lot of the THC will have broken down into CBD and CBN by then, giving you sleepy/relaxing/couch lock effect and not the mind expanding cerebral head high you are after.
Hope this helps, Organoman.