The plus systems are completely different than the two rail shots in Beards book. The carom systems are based on using running english, and they don't match up with hitting the object balls on the same tracks. Object balls will go much longer hitting the same spot. The carom tracks are good for kicks, are a little tricky regarding how much english you use, but are good to know.
There are other systems basically the same as the plus 2 systems for shooting the object balls, but the "plus" numbers are much different.
I don't have my copy handy, but I'd guess Beards' books go over how to estimate the hit point between his diagrammed examples. Speed into the rails and the english on the ball greatly affect this shot,too, so memory (feel) might still be the best method.
There is a discussion of this very thing on onepocket.org that popped up just this week.