This assumes a linear correlation between labour spent and gross income earned. The tax brackets are known to everyone, including those who pay the salaries -

This assumes a linear correlation between labour spent and gross income earned. The tax brackets are known to everyone, including those who pay the salaries - if you want to get someone to do that 20% extra work, and they'll want a 20% increase in net pay, then you can just do the math and pay the appropriate gross.