We told you about the Cancer Council's SunSmart iOS app almost a year ago. A recent update to the app adds a sunscreen calculator, suggesting how much sunscreen you need based on your size and the amount of clothing you're wearing.
As a rule of thumb, you should be generous with sunscreen: full body coverage for a typical adult requires around 35 mls. But if you want more exact figures, the app is a neat way of finding out what you need. It's a free download for iOS users.
SunSmart [iTunes App Store]