If you don't have a pocket microscope to view the trichomes, then I found an old schooltrick that will help you.
Flush a week after you see almost all the pistil's turned red and there is not much if any new pistil growth. Once pistil's stop growing...you're done.

trich's are the best way, but this will get you in the ballpark.

also, you can go to far!!! Once a plant is at it's optimum potency/maturity and the pistil's stop re-producing...The thc starts to deteriorate...i.e., once a trichome turns amber...It is on the down side, deteriorating.

Imho, It is better to Harvest on the early side for a more heady-up-high...All dark trich's mean 'couchlock/coughing reefer.

As knowmy said; Don't flush yet, they look great...shoot the nutes too'em.lw