Tiered Commission Calculation

I am trying to develop a way to calculate commission based upon a sales goal percentage.  For instance, if the sales person meets less than 50%, they would receive 0% of Margin.  If the sales person receives 50% they would receive 10% of Margin.  If the sales person receives 65% they would receive 20% of Margin.  If the sales person receives 80% they would receive 30% of margin.