For practical purposes, the useful in-betweens that fall within each quarter alignment are typically no more than 1/8 of a ferrule in aim/alignment difference.
In other words, using the width of the ferrule as a guide/gage, one can easily learn to aim 1 to 2 degrees thinner or thicker than the nearest known aiming reference.
A 1/8 of a ferrule aiming difference is about 1.5° for shots around a 1/2 ball or thicker. It starts changing quite a bit as the cut angle gets bigger. It's a 2.5° difference for shots that are closer to a 1/4 ball.
Anyway, using the width of the ferrule split into eighths can help a fractional player define/pinpoint about 20 different cut angles for shots between 0 and 30 degrees.
That's plenty accurate enough to pocket most shots, and plenty defined enough to help a player develop solid aiming skills (the visual knowledge that allows us to feel/know how to aim or align any given shot).