Over thinking it is the point! 
Trackrash,
Thanks a lot for your explanation. I asked the question because I had seen two different ways to use vacuum discussed, and I wasn't sure which scheme Porsche was using.
While reading about the
magic SVDA distributor for VW it clicked that MAP is a pretty good approximation of engine load. High manifold vacuum, low load. Low manifold vacuum, high load. A vacuum system that is intended to advance timing at high vacuum conditions gives more mileage and maybe lower emissions, probably runs a bit nicer too... I would be interested making this kind of system.
The other kind I've read about was part of the emissions malaise awfulness that infected engineering in the mid '70s to early '80s. In these systems the vacuum is used to reduce advance at idle and I think the system tends to reduce overall performance. Looking at the vacuum curve from Bosch, I read it as increasing vacuum reduces timing by up to 10 degrees. This may help with idle, but my distributor can be electronically set to give 5 degrees ATDC without having any impact on total advance. It also means that in lightly loaded conditions, highway cruising, its actually removing advance. (This assumes I'm reading the chart right.) If this is the Bosch design on the stock 2.4, then I'd plug the port and never think about it again.
The engine I'm working on is a stock '72 2.4E that formerly had MFI, type 911/52. It's a 8:1 engine - nicer than my old T, but still not in great danger of detonation. I'm going to run it with Zeniths.
I think I have the vacuum port issue worked out. The Zeniths had an idle enrichment system that the majority of people immediately disable. (I'm no exception) This leaves me with a set of ports significantly below the butterflies that are unused and should provide MAP. My plan is to run 3 into 1 and take the reference from only the left side. If that's not good, then it's just a matter of plumbing to get the other 3 involved.
To RS Targa,
This distributor I'm working with is a nifty device. All I need to do is give it an accurate TDC and it figures the rest out from there. If I feel so inclined I can program it to give 45 at 6K, and still have -5 at idle. (That would likely break something...) The point is that you can change any point along curve without impacting the others; I'm completely free to program any curve I want, and that I can use or not use vacuum to modify it.
It's almost too much freedom.
So, if you could run any curve on a relatively low compression motor, what curve would you run? Would you use vacuum?