No you want to time the magnesium levels perfect, you want the plant to have used up all magnesium 2 days before harvest. I feed molasses to my plants, and my last feeding is usually 3 weeks before, then maybe, just maybe, do half the amount I normally do with 2 weeks left. Magnesium is what makes your smoke harsh. As far as the ppm goes I couldn't tell you, soil and stuff. but yeah, usually want all metals flushed 2 weeks before. If your leaves are yellowing with 2 weeks left, you stopped feeding too soon. If they start to yellow with 7-10 days left just keep giving it water to flush that shit. Idk, that's what I know about soil at least. I'm sue the magnesium levels should be the same towards the end