Having gone through the pain, read many articles & advice, and talked to fellow refers I've come to one conclusion: Nobody knows with absolute confidence how to deal with ich! I also learned that doing nothing (other than research and trying fresh water dips) leads to catastrophy.
It seems that copper is the most reliable method of treatment based on my own research, so you're probably off to a good start. IMO I would continue with treatment and follow the instructions exactly as written.
I bought a powder blue tang a few weeks ago (perhaps the most suseptible fish to ich) and did a 1 week QT with coppersafe for good measure. He did fine for a week in my well established 125G, then came down with signs of ich. He was being bullied by my adult domino damsel, which combined with his new environment, probably triggered the ich. I netted him out and he's been almost 2 weeks back in QT with coppersafe. All signs disappeared after a few days, so it seems to work well. I'll keep him in for another week before I get him back into my show tank.
At any rate, IMO I would continue with cupramine as directed and not get caught up in measuring the amount of copper in your QT. It has been tested at the amounts indicated so I can't imagine it would create too much copper if administered as indicated.
I would however make sure you are checking your pH, nitrites/nitrates and amonia levels in your QT! Assuming it's not an established system so you will get serious spikes (especially with that much of a bio-load. Best of luck to you!