How do you measure underbust JSKs?

This is the JSK in question. It looks like it will fit any bust size because it's open. I could be wrong because I have a smaller chest and can't always relate to how things fit normal/heavy chested girls. I'd like to sell it but don't want to steer anyone in the wrong direction with a bad measurement. In lolibrary, it says that the biggest it can fit is 82 cm, but after having tried the dress on, I know that's wrong as brand measurements often are. Can someone help me out?