If your in gear idle vacuum is 12 use springs for 6 in hg to get you close. EDIT for max mileage you might mount a large vac gauge up close where it is easy to monitor so you'll be aware of what any amount of throttle shows on the gauge and knowing at what vac the springs kick in. If your not going to be into the power range any set of lighter springs that will let the metering rod tree stay pulled down & on the "cruise" step would be a good plan. I mounted a large vac gauge on my steering column & it was the best thing I ever did.