I use the metric system, so I am going to explain with transitions as well.
I know 12 (f)ounces is equal to 355ml.
250 ml makes up a cup of liquid.
250 ml is 8.45 ounces.
So if 8.45 ounces is a cup of liquid, and an extra 3.55 ounces is about 3/7 or rounded to 2/5.