I'll bet somewhere in a store is a bottled drink which says "8 oz" (probably imported) Why don't you buy one of those, drink it (or dump it out) and then fill it with water and pour it into your own measuring cups to figure out what it's equivalent to?
However, that's getting nitty-gritty, I think approximating is fine, people generally increase or reduce the oil dosage as they do different flushes. Smaller people don't always need as much oil, but some people find they need a much higher dose of oil to get results. It's all an experimental game so as long as you know what amount you use, you can adjust it in subsequent flushes.