The 80:20 rule is based on the assumption that there is (often) an 80% (ish) subset of the realised value for a project that only costs 20% of the resources (e.g time) to complete. The rule, actually the Pareto Principle popularised by Richard Koch and others, like all rules of thumb, can be applied incorrectly.
It's not a license for doing things half-arsed.
