Summary fields are good for calculating totals etc at a summary or sub-summary line on a report. But I can't figure out how to do a "cross-calculation" using two summary field figures on the same summary line. Any help appreciated! (I'm using FileMakerPro v10 -- at home, basically for only a couple of applications, so not a big user).

You can just make a calculation field that adds the two summary fields together. Make sure it is unstored. Alternatively you can do almost any summary calculation with ExecuteSQL.