I treat it like tobacco. There are a few things that I use. If the plants are seeded, when the pistils have fully retracted ya might as well just use water. If they aren't seeded I use the leaves. When upward and outward growth stops and there is no new upper growth using nitrate or nitrogen products is a waste. Once the nitrogen which is basically sugar to a plant is used up it begins to yellow. When the plant looks 25-50% yellow and its been growing awhile then ya night as well feed straight water. Its pointless to feed a dying plant. I use organic nutes for flower though so I don't flush like some do. Or I don't use the same method rather. I don't have high salt buildup like you would encounter with something like hydro.
As an additional note and I'm not saying its wrong nor an I trying to step on anyones toes but counting on the leaves to fall off to determine the time to flush or harvest isn't all that accurate in most conditions. A young healthy plant will lose leaves due to lack of light penetration so unless you've grown out a nice tall sativa dominant plant with all the light possible you're probably going to lose leaves prematurely. For those with experience me saying this is stupid but for others that are not it will make perfectly good sense. Learn to judge the plant by its overall looks, not by whether a leaf falls off or one gets some yellow spots. know what you're growing and when the plant doesn't look as vibrant and full as it did its time is done.