How to create a calculated PDBA from another 2 PDBAs

Hello,

In JD edwards enterpriseOne Payroll:

I need to have a calculated Benefit PDBA which is Equal to "Salary" minues "Social security".

example:

Taxable PDBA = Basic Salary - Social Security.

If an employee has basic salary 1'000 USD and the month social security is 200 USD, then the Taxable PDBA should be 800 USD.

I will then put this taxable PDBA as a basis of calculation in the "Tax Income" deduction PDBA.

I guess i can use P07901, but i am not sure of the parameters to put

Thank you,

Norma