By the way,

I just managed to pick up a neat old Snap On vacuum gauge still in its original box with instructions for 15 bucks on Ebay!
When I get it, I'll measure the vacuum at idle when warmed up. Then, as Pat says, divide the reading in half and use that figure approximately to pick the correct power-valve.
A friend over here says he jazzes the motor and sees what the vacuum drops to, and then picks a power-valve just above or below that figure (I can't remember which). Does that work too?
Chris