If you approach the problem from the standpoint of, 'How much oil do I put in,' (volume) you don't take into account what's already THERE. Meaning, how much you didn't get out, didn't drain during your fork service.
Make yourself up an oil level tool....a turkey baster, a horse syringe with a piece of tubing on it, simply a couple pieces of paper used as dipsticks.
Set to suit. As larrk says, you'll find a lot of opinions on the matter. The more oil (higher the level) the more firm the fork action (less air to compress, more oil that doesn't compress).