"Im also wondering how one knows that they have supplemented enough zinc,"
I think some in the FDC group use a taste test, but we don't hear about high zinc issues when people are following Cutler's dosing suggestions.
"...when does copper deficiency become a threat"
I think that is one of the few times where Cutler recommends followup hair tests. You'd have to check his hair test book for more on that--noamalgam.com.
I don't recall hearing any reports of this in the FDC group. It seems fairly rare to hear about low copper problems. Hi copper seems to be the much bigger risk.