runoff can be a bit higher than what is actually in the substrate. If you are religious about your drain-to-waste runoff, like growerofthemonth mentioned, all is well. (it will maintain equilibrium if done properly)
if you haven't been consistent with it, it could be a cause of elevating nutrient levels in substrate. Plant doesn't dictate any pressing need to light your hair on fire.
flushing is for imminent catastrophe... A little extra runoff after a slight formula change should be sufficient to shift contents of substrate immediately unless it is a crazy drastic shift in ppms for any 1 molecule of need.... then, it may take a couple irrigations before new equilibrium is reached. Can always water with a little extra runoff beyond normal, too... just avoid a volume that causes plant to droop (overwatering). tht will mainly depend on how well your substrate drains ( % perlite etc).