People usually wait for a bit of amber to start their flush. With that in mind though there was a study done that showed flushing the last 2 weeks doesn't do a whole hell of a lot. If you wanna save nutes just feed half strength. https://atrium.lib.uoguelph.ca/xmlui/bitstream/handle/10214/12125/Stemeroff_Jonathan_201712_Msc_with_erratum.pdf?sequence=8&isAllowed=y pages 58-60 show graphs that show nutrient comparison levels. overall it was a difference of less than 5% between flush and no flush which isn't enough to change anything.