I've looked at this before (for Megasquirt, not 123, but they're similar enough), and the curves in the ^^^ greenbook need some explanation. I think I have this right, but long ago gross-polluter (Nick) couldn't get his analysis to match reality.
Here's the curve for the standard B21FT K-Jet distributor:
1 - the top scale is for centrifugal advance, and is in terms of Distributor/Cam revs per second/minute. Double this number for Crank/engine RPM.
2 - use the outer degrees scale for Crank timing degrees
3 - the bottom scale is for vacuum advance and boost retard. It's relative pressure versus ambient, e.g. 0 with engine off. Add 100kPa to get absolute pressure assuming you're at sea level.
4 - this pair of lines is the centrifugal advance, with the lines showing min/max due to mechanical tolerances. This is added to the base/idle advance, normally ~12degrees. For example, 1500 cam rpm adds ~18degrees, or 30 total.
5 - this region and pair of lines is the vacuum advance for negative relative pressures. It's added to the base+centrifugal advance. For example, -20kPa (or 20kPa of vacuum) adds ~ 5 degrees.
Note: the original setup uses ported vacuum (opening just outside the closed butterfly) so that there's no vacuum adder at idle. As soon as the butterfly is cracked, vacuum advance jumps to near max vacuum.
6 - this region and pair of lines is the boost retard for positive relative pressures. For example, 25kPa of boost retards (subtracts) timing by ~ 5 degrees.
