Back on track then....
The article unfortunately ignores anything to do with how efficient your speakers are... You plug the same power amp into two different makes of speakers at the same settings, and you can get two completely different volumes...
Matching the power amp to the speakers is more important, IMO, than any vague formula for power/customers (that doesn't take venue acoustics, type of music usage and ambient noise into account).
IMO, an overpowered PA is no better than an underpowered one. But a PA that is balanced, but considerably more powerful than you usually need is still very good to have, if it doesn't become too heavy to use. Then, when you get a call for a MUCH bigger engagement than you usually do, you don't have to use unfamiliar equipment (and risk sounding less than your best) to do it. Or you get a call for a full band gig, you won't tear up your little Logitechs trying to put a live drummer and bass player through it!
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An arranger is just a tool. What matters is what you build with it..!