You want it undetectable. You might find with the hanna checker you see .01-.05ppm which is fine. The tester has a variance of +/-.05ppm. Any higher than that I would try to run carbon, cuprisorb, etc. To remove any.Thanks.
I have one additional question. I was wondering what the acceptable range of copper is in a reef tank? Ideally we want zero I understand but I was wondering at what level is it starting to get bad for corals and inverts. Just checking since I am using distilled water right now for my tank top offs.