Girlfriend, you're right on track. Half a yard would be 18", which is not enough. I always buy more than I think I will need to allow for crooked cutting and my mistakes, so I would up the yardage to 3/4 of a yard (24"). Although 5/8 of a yard is adequate (22.5"), it doesn't allow for mistakes. I always go up a little more and add any scraps to my stash.
If you have a calculator, an easy way to figure out how many inches is in 5/8 of a yard is to do this: 36 times 5 divided by 8. The result is the number of inches in 5/8 yard. This works for all fractions because a yard is always 36.