To create a new allowance and compute it as an additional CPF, please follow the steps below:
Go to the Payroll module, Pay Element Setup and click New.
Select Allowance and click Next.
Select None of the above and click Next.
Select Standard Allowance and click Next.
Select Free, and click Finish.
Remove Subject to Ordinary, arrow over from right to the left.
Select Subject to Additional, move from left to right and click Save