Is there a saying (ことわざ）or 4 character phrase (四字熟語) in Japanese for the 80-20 rule (i.e. the Pareto Principle)? i.e. it takes 20% of the effort to get 80% of the results but to get the last 20% of the results takes 80% of the effort.
What you describe is also known in English as the Pareto principle.
From this site https://takuminotie.com/blog/2015/09/09/post-9226/
we have the usage
The site also says that...