In summary, it can be hard to know what the 80% of things you should concentrate on actually are.<p>Especially in startups and certain endeavours (e.g. Marketing) it really it is a case of continually trying a lot of things to see if something works.<p>Once you've discovered what works, you could focus all of your time on that one thing. The problem then is that you have no further opportunities to discover something that is even better. You may have hit a local limit.<p>So you need to keep trying new things, and potentially wasting time on them, until you can evaluate properly to see if they move to the 80% of things to focus on in the future.
"The 80/20 Rule would recommend that the most effective course of action would be to focus exclusively on serving these clients (and on finding others like them) and either stop serving others or ". Whereas my understanding is to know which of your clients are regularly contributing to your profits while also trying to improve profits coming from remainder. Those in the remainder may appreciate your help, improving your future results.
Honestly, this is where one of the old cliches of go where your heart takes you, works really well. Your heart will tell you if you need to volunteer or start a business or do something else. And you should listen to it, probe it a little bit and see why it wants that.