While I wait for my Dart's rear end rebuild, and I'm out of town anyway, I was wondering about cruise vacuum readings since I plan to start playing with power valves and PVCR diameters.

My 451 has a 272 @.050 mushroom cam, and idles at 7" vacuum at 1050 rpm. It'll be turning about 3000 rpm at 60 mph (295/50R15 tires, 3.91 gears).
Any idea what vacuum will be in a level cruise at 60 mph? work
Or is there no way to predict it, other than actual measurement?