Aww, come on, you've got come up with harder questions than that!

All you need to do is enter 10 into the "Target Gravity" second text area (the one for the %ABV), then click somewhere on the form or press the tab key, and the SG will be updated to 1.080

The SG and the %ABV are linked together, so entering a value in either one will cause the other to show its associated value (just click on the form somewhere to trigger the update).