A true flush is using 3x the amount of soil you have, but in water. So, if you are growing in a 5 gal pot and you want to do a flush, you need to flush with 15 gallons of water lol
Therefore, I’d pH all my water to 6.5 because by the end of 15 gallons, you should get the soil pH to 6.5, but that’s also why I say to be careful and keep checking the runoff after each gallon you use during the flushing process.