SPM171400 | Average Weekly Earnings (AWE) - SMP/SAP/ShPP/SPP/SPBP/SNCP: no fixed pay period
From HM Revenue & Customs · Statutory Payments Manual
If an employee does not have a fixed pay period:
find the date of the last payment to fall on or before the end of the QW/MW/ONW/RW to establish the last day of the relevant period
find the last payment to fall at least eight weeks before that and come forward one day to establish the first day of the relevant period
add together all payments in the relevant period
divide the sum by the number of days in the period and multiply by seven to obtain the AWE.