If you have 300 or fewer employees, and have experienced a 25% or higher reduction in gross revenue during any one calendar quarter in 2020 compared to the same quarter in 2019, then you use this formula:  Take your average monthly payroll costs for 2019 or the most recent 12-monty period ending prior to the date of the loan application (higher of the 2).  Divide this number by 12 and multiply by 2.5.  Those in certain hospitality fields use a multiplier of 3.5.  This will be the loan amount.

You have to cap all salaries at $100K per employee, and there might be a few more limitations.  Sole proprietors will use their Sch. C net profit ($100K cap) in place of payroll costs (plus wages paid).  You can also add certain employee benefits and taxes paid to the wage amount.