Vacuum advance tuning to maximize cruise MPG?

You really need to have efi with 3D fuel and ignition tables, to optimally tune. Then you can monitor things you need to know when tuning. Important things like RPM, MAP, ignition timing, fuel pulse width, AFR, and temperatures.

You may find with the mechanical advance distributor, too much timing advance when coasting down at light throttle, results in a lope. You may also notice ping at light throttle that goes away if you get into it. Connecting the vacuum to manifold may result in run away idle return, and other problems.

The adjustability of the vacuum advance can be monitored using timing light and mightyvac. Hook the hose to it, apply vacuum, then record and plot timing/vacuum. You will find where the advance starts, stops and slope of the ramp, plot that. Then carefully turn the adjustment, see again what that does. If you are good with trig math, and measuring, you can scribe marks on advance plate and distributor, or use dial indicator and measure degree travel of advance plate vs vacuum. Crank degrees is twice that.