Benefit salary for insurance plan basic pay + additional payment

The help on Benefit Salary value says the following:

The estimated annual gross salary of an employee. This salary forms the basis for several calculations in Benefits Administration.

The system determines the benefit salary by reading the following employee master data records on a particular date, or within a particular period:

Basic Pay (0008)

Recurring Payments/Deductions (0014)

Additional Payments (0015)

The system reads the valid Basic Pay (infotype 0008) and Recurring Payments/Deductions (infotype 0014) on one of the following dates:

The calculation base date (default)

A date calculated from the cutoff day (override day for cost and credit variants only)

The system reads all Additional Payments (infotype 0015) that are made in the year preceding this date. The sum of all these amounts is then multiplied by the number of payments the employee receives during the year from the period modifier on their Basic Pay record (infotype 0008).

I have set up life insurance and want the coverage amount to be equal to the employee's salary plus the additional payment from infotype 0015. How do I set up the coverage rule to make this happen? If I add a salary factor on the coverage rule it seems to just take the salary from basic pay infotype.