How can you? One is taking about 80% of the work and the other is talking about half the work. You would need a distribution of the work impact across the employee population first.

The Pareto rule (at least my interpretation, due to its similarity with a power distribution) is recursive: 80% completeness from 20% work also implies 64% completeness from 4% work which in turn gives 51.2% completeness from 0.8% work (half of success is just showing up).

Accepting 51.2% as an approximation to half, the problem becomes 0.008 x = sqrt(x) which gives x = 125^2 = 15,625