Use one of those glass measuring cups from the kitchen. They all have ounce metrics.
I used copper power many a time before I came to this site that suggests hyposalinity. I had real mixed results with copper power. Mostly, it may reduce the signs of the ick but once you start to take it out - here comes the ick. Also, putting in the suggested amount will not cure the ick. You'll need at least twice what it suggests. I quit using it and started looking for a better way cause I ran double the dose for double the time and still had the same problem.... It seemed that the ick could acculmate to the level of copper. I wont use it again.
Now I am doing hyposalinty, took the dog puffer out of the main tank and put it in the QT. I've had no results, measuring with refractometer. 2.5 weeks in and ick is very visable. I am considering using hypo and copper at the same time? Thoughts on that? Also, what is the best copper treatment to use and measuring kit to go with it?