You need to check trichs under magnification. You can pick up cheap used school microscopes which are the best. Or those little pocket microscopes, or just a 30x or greater loupe. Look at them on samples trimmed from several places around each plant and see how many are amber. Sort of take an average and wait till it is up to 25% or more. Keep in mind that far more growers harvest too early than too late. When in doubt, wait.