It's all opinion on when and how to flush.
First off you need to guesstimate when your harvest will be, some say flush 3 weeks, others like myself 1 week. Some just water normally for a few weeks, others actually bring their plants to a tub or sink and flush with 10+ gallons of water.
I only flush a week, even if you heavily feed in peak of flower you need no more then a week.
If I'm in a 5 gallon the first flush I will run as much as 10 gallons, but I usually end up doing as little as 6. Then the next watering about 3-4 days later depending on environment I do the same amount of water as the size of the container, so 5 gallons. Then the last flush I do a little less then the 5 gallons, maybe 3-4. Then I allow soil to almost dry out and then harvest. In my environment it's about a total of 7-10 days.
Water only. No need for flushing agents.