It is up to you, the customer to accurately calculate the average monthly payroll for your PPP loan.
According to the Treasury, "providing an accurate calculation of payroll costs is the responsibility of the borrower, and the borrower must attest to the accuracy of those calculations. Lenders are expected to perform a good faith review, in a reasonable time, of the borrower’s calculations and supporting documents concerning average monthly payroll cost. The level of diligence by a lender should be informed by the quality of supporting documents supplied by the borrower. Minimal review of calculations based on a payroll report by a recognized third-party payroll processor, for example, would be reasonable. If lenders identify errors in the borrower’s calculation or material lack of substantiation in the borrower’s supporting documents, the lender should work with the borrower to remedy the error."