For employees whose pay does not vary, this is their ‘regular’ salary before tax including fees, compulsory commission payments and past overtime (although the expression ‘non-compulsory overtime’ has slipped into the latest guidance) in their last pay period before 19 March 2020 but excluding discretionary bonuses (including tips), commission and non-cash payments. If employers have already calculated payments based on previous guidance up to 28 February 202 then this figure should be used.
For those whose pay varies it is the earnings in the same pay period in the previous year; or the average earnings in the 2019-20 tax year (or fewer if they have worked for less time than this, including a part-month calculation if they were taken in February) whichever is the higher.
https://www.crosslandsolicitors.com/site/cases/coronavirus-job-retention-scheme-Q-and-As