Hi Spaceman,
See "Brake specific fuel consumption explained":
http://ecomodder.com/forum/showthrea...ned-30791.html
Other searches in this forum will also be helpful. The best search terms are "specific fuel consumption" or BSFC.
One thread has a series of BSFC curves for various cars (but unfortunately not our Duratec Foci). Generally, the best rough guess is -- I think -- 65% to 80% load. There can be hidden sweet spots in the curves, and RPM is also a factor. For most of us the 1,000 to 2,500 range will be the most important.