Add field1 as a measure in your summary. You'll see SUM(field1) as the calculation.
Now double click the field1 line in the measures box. This opens a dialog box with three tabs. Click on the Calculation tab. Now click the Edit button and change the formula from SUM(field1) to be:
[font="courier"] (SUM(field1) * 100)/ SUM(field2) /font[/quote]The name of the field will have changed to be RATIO(field). Click the general tab, uncheck the "Use default title" box and give the field a more meaningful name.