I'm not sure where I stand on the PK thing yet. This is the first time I've used "cannabis" nutes, so we'll see. But I do believe most peoples grows on here I've seen drop the N way too early, and looking at the growth I got from FF I understand why. It took me one fertilizing to tell something wasn't right when all the new leaf foilage was yellow-green.
There's nothing wrong with some leaf on the plant in flower, in fact it's beneficial. The idea isn't really to overload the plant with N as foilage production on it's on is going to decrease, but you do want to make sure any new foilage is as healthy as possible, and the mature foilage maintains it's green.