You take the shaft weight of 4.2 ounces and divide it
by the cue’s playing weight that we know to be 20.2 ozs. (16 oz. Butt & 4.2 oz. Shaft). You get 0.0279 or the %
equivalent of 20.79% which can rounded to 20.8% weight proportionality. The shaft represents almost 21% of
the assembled cue’s playing weight. As I tried my best to explain, there is a rule of thumb approach used by the
best cue makers striving to match a shaft to a cue butt to achieve the best hitting combination. This is just a fact.
This is absolute hogwash and does not provide a "best hit", as if such a thing even exists. It does however provide the weight and balance point that many people prefer (and that some others don't) based on their personal preferences.
Many cue makers make heavy shafts for these reasons:
1. It can be used as a selling point. They will claim they are superior for reasons that sound intuitive to many people, and many people will buy into because of that, but it is actually nonsense that is not supported by the evidence.
2. More people than not, especially in recent decades, prefer a cue with some level of "forward" balance, and you can't as easily make a forward weighted cue with a very light shaft. There is nothing inherently superior about a forward balanced cue though. Whatever balance point a person most prefers (whether rear or forward) is the one they will ultimately shoot the best with, although within reason people can get used to just about any balance point and shoot at 99+% with it if they give it the time.
3. People became convinced that old growth tight grained wood "hit" a certain way that was superior, and those shafts are usually heavier (which also led some to falsely conclude that heavier shafts hit better). So even when a cue maker wasn't using old growth tight grained wood for their shafts, they still wanted the shafts to at least be heavy so it wouldn't be so obvious and so they wouldn't lose that portion of the market. The truth is that old growth tight grained wood shafts (or heavy shafts for that matter) certainly don't inherently
play better than anything else. As far as their "hit" goes, "hit" is essentially just the exact type/feel of hand shock you feel in your cueing hand at the time of tip contact with the cue ball, and there is no one type of "hit" is superior to any others. It's just a personal preference type thing. There isn't even any good evidence that anybody would be able to tell the hit of an old growth tight grained shaft from a similar shaft that was using standard grained wood if they were blindfolded, but even if they could it is still just a personal preference for the type of hand shock/"hit" they want to feel in their hand.
4. Many people (like you) became convinced, wrongly, that heavier shafts "hit" better, largely because of guys you like professing such nonsense constantly, and when people hear something passed around a lot as the gospel truth they often buy into it themselves and then it continues to multiply, and now here we are with lots of people having silly beliefs. Heavier shafts do not inherently "hit" any better or play any better.