I would not flush with 8.5 personally. Even if your soil pH is low enough to cancel it out, I would not go above 8, and if your soil pH is close to correct, you would end up knocking it up out of range.
Just a pH tester and a bottle of pH down to match your flush water to the observed runoff (for example if the runoff is 6.0 you can flush with ~7.5 to bring it up; if the runoff is 6.6, flush with 6.6 because you are already dead on the money). And don't forget that the last step in your flush should be to water with a 1/4 strength complete fert solution.

Another thing you can do to 'cheat' is use the corrected tap water for the bulk of the flush, then do the last bit -like the last gallon or so- with distilled or RO; again, feeding lightly afterwards.
stinkyattic Reviewed by stinkyattic on . Flushing Here's one for the experts. Depending on what happens in the next few days I may have to flush 5 plants. The standard from what I read is 3x the amount of water of the gallon you are using. I am using 3 gallon pots so that would be 9 gallons x 5 plants = 45 gallons. I would need to borrow a truck to get that much water home. I have always watered them with RO water from the store in 5 gallon increments. My tap water is 8.5, way too high it seems for plants. Any suggestions on how to flush? Rating: 5