Check the EC of your coco by floating it in enough water to cover and letting it sit for 15 minutes or so. If it's close to what your water is then consider it flushed.
I don't worry about the pH of my runoff, the medium controls that, all I worry about is the pH and EC of my nute solution going in.
In the event that you haven't picked up an EC meter yet, it sounds like you've flushed it more than enough. An EC meter isn't an absolute must, but you'll be happy you got one.
