Flushing is only needed in extreme cases of eff ups, such as feeding waaaaay too much or if there are problems with toxicity.
It is not needed as a general "just because", it will just kill most of the beneficial microbes in your substrate, leading to plant stress.
"Flushing" at harvest time is not an exercise in drowning the bejeezus out of your plants, but an exercise in nutrient with holding so that the plants get rid of any residual fertilizer salts from their vascular system. All that is required is to give plain water, nothing else, in normal quantities, at normal intervals for the last 7-14 days before the big chop chop.
Flushing should rightly be renamed "nutrient witholding period" to prevent ongoing confusion regarding this operation.
Since you are growing in coco coir, either "run to waste" or ensure there is run off each irrigation and this will prevent nutrient salts from building up to toxic levels in your substrate, negating the reason for ever needing to flush.
Hope this hellps, Organoman.