Well, if the trich's are starting to turn milky i'd wait untill about 25% were milky, and the rest clear and then i'd start to flush the plant and stop feeding anything. Then i'd just let the plant carry on with just water untill the trichs we're coloured how i wanted them. Bascially, once almost all of them are milky your at optimum and its your judgement from there on. Have you decided what kind of ratio you'll go for before harvesting?