The reason most factory setups use full manifold vacuum at idle is to smooth out the idle, lower emissions and have the engine run cooler. I tried both settings, manifold and ported, and decided that manifold vacuum worked better for me. Believe it or not, using manifold vacuum tamed the idle from full beast to tamely aggressive, reduced noxious exhaust fumes, and reduced coolant temp at idle by nearly 10*. Go figure. My understanding is that the factories added vacuum advance at idle primarily to reduce emissions at idle. The added timing didn't harm the motor because there is no engine load at idle. As soon as you step on the throttle manifold vacuum drops which removes the vacuum advance (usually 22-24* built in from the factory). In summary, assuming 10* initial advance using the timing light, adding the vacuum advance at idle gives a total advance of 34* which drops off rapidly to 10* when the throttle is applied, building back up to 36* at high RPMs when the centrifugal advance kicks in.
Still confused? Try reading this article:
http://www.fordmuscle.com/archives/2...ng/index.shtml