The First Pinnacle covers the opening stretch of a life, from birth to a changeover in the subject's twenties or thirties, and it is therefore the only position in the chart routinely read entirely in retrospect.
That changes what a reading of it can do. A pinnacle still to come is offered as guidance and can in principle disappoint. A pinnacle already lived is offered as an interpretation of what happened, and the client supplies the material. Told that the first three decades were governed by a four and characterised by constraint and steady effort, most people can find enough constraint and effort in a childhood to make the description fit, and the ones for whom it does not fit tend to conclude they were resisting it.
That is not a criticism unique to this system. Any retrospective account of a life works the same way, and the reason has been studied under several names in psychology. It matters here because the First Pinnacle is often the position a practitioner opens with, precisely because agreement is easy to obtain, and the agreement then lends credibility to the prospective positions that follow.
Within the tradition the position is used more constructively than that suggests. Practitioners read the first pinnacle against the later ones to describe a trajectory, and the interest is in the change of number at the changeover rather than in the first figure alone.
The arithmetic is the reduced birth month added to the reduced birth day, reduced again.