Go with double or triple the size of your pots for the amount of water to use. Keep watering till it runs clear. If you have a ppm meter you can test the run off too.
When I flush my plants I pour 1/3 - 1/2 gallon in and then let then tip it to let the water drain out of the tray the pot sits in. I do this 2 or 3 times. Depending on the situation. I'm not expert but it seems like this method works.