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.