flushing is pointless unless it needs to be done... why are you flushing?? do you have some issues with your plant??
unless your plants are suffering from lockout problems, which would be PH problems related to salt build up in your soil, flushing is a waste of time and energy..
the only time it really needs done is towards the end of flowering.. and even that is subjectable to which grower you talk to...
when i used to grow in soil i would run 2 to 3 times the volume of the pot of 1/4 strength nute solution through... the 1/4 strength solution will remove more salts that have built up than plain water will.... and this also ensured me my plant wouldn't have, or at least it would minimize, any deficiencies that may set in during the flush...
i would do this once, and if needed, repeat a few days later...
when it came time to harvest, in the last 10 days, i would flush 2 times with the 1/4 strength solution, and then my final watering the plant would get would be a flush of plain water....