Short answer: The easiest way to calculate sales staff pay accurately is to stop calculating it by hand. Sales compensation software — also called sales commission software or incentive compensation management — automates the calculation, tracking, and payout of variable pay for sales teams by reading deal data directly from the CRM, holding the compensation plan as structured rules, and keeping a record of how every payout was produced.
Where commission is still calculated by hand, the process has a consistent shape. Someone exports closed-won deals from the CRM into a spreadsheet. They apply the compensation plan manually. They produce one number per salesperson. That number is typed into payroll.