The 80:20 rule is a heuristic that people use to suggest that 80% of the benefit comes from 20% of the effort (the values of the parameters may change, but the idea is that a relatively small share of effort gets a majority of the benefit). In one sense this is the idea of diminishing marginal returns, as the last 20% of the benefit requires 80% of the effort. This idea is related to the Pareto principle.
If this is true, the question arises: is the 80:20 rule recursive? Of the first 80% of benefit, does 80% of that only require 20% of the first 20% of effort? In other words, is there a 64:4 rule. Or worse, is there a 51.2: 0.8 rule. If so, then less than 1% of effort gets more than half the benefit.
That sounds like a really good deal to me.