manifold vacuum isn't necessarily better, but with a given set of parts and low vacuum to begin with somebody may not have a choice. i always use ported but who am i to say manifold vacuum is off the table. i have one set-up on one of my cars that uses a 9 degree plate with 36 degrees total. i use a light primary spring that begins the centrifugal fairly quick and idle is about 23 degrees even though i know the initial is 18 degrees. my other car won't tolerate this kind of timing and this is why i believe that curves have to be individually set-up per engine/car/driver. i think it wouldn't be easy to buy something from a vendor, even if they have all the info, that will be perfectly tailored for an individuals combo.