As long as it's between 6.0-7.0 you're good (6.4-6.6 being about optimal)... but if you're tap water is 7.0 going in, and your run-off is 6.0, that means your soil is somewhere near 5... which is too acidic. Anyways, you could flush again (same day it's probably ok) or just wait for it to dry out and test the ph again next watering. Don't want to over-water by flushing too many times...