I was asked by several applicants about the titled topic during interviews and made an inquiry to the secretariat of the monthly financial support.
Their answer may be helpful for individual business owners earning various monthly incomes since before the spread of COVID-19.
If you filed blue form of the final tax return, you can choose by yourself the single monthly sales amount of the target month or 1/12 of the annual sales amount of the target year.
If you filed white form of the final tax return, you cannot choose the single monthly sales amount of the target month for comparison.
If you are not sure with amount of the support and whether you are qualified or not, you can access the page “the monthly financial support simulation” in the portal site and check your receivable amount at https://ichijishienkin.go.jp/getsujishienkin/simulator/kojin.html