Well how much water is correct amount brewer!?
The amount of water needed for flushing depends on your soil's composition. If your medium drys out quickly and you have incredibly good drainage, then dump a lot of flush water through your pots. Maybe 3 times the volume is fine in that case. At least dump as much flush water though as you have soil in your pot. But if you're like me and only have about 10% perlite and 40% vermiculite, the medium tends to hold water for several days before it needs watered again. In my case, I flush with the same volume as I water with but do it for the last 2-3 waterings.
How can you over water when your pots have holes in the bottom?
Your medium holds water and while excess drains out the bottom, completely and totally saturating your medium can be bad for your roots if your soil isn't set up to drain that kind of volume.