Unable to use IV_START_DURATION and IV_END_DURATION in Global Absence Type Duration Formula

I have created a new compensatory off plan and absence type with UOM as hours. 'Partial day rule' is enabled, however we have a different requirement for duration calculation.
I have written a basic FF of category 'Global Absence Type Duration Formula' with inputs as IV_START_DURATION and IV_END_DURATION. When I am attaching this Formula at the 'conversion formula' placeholder at absence type level, it is automatically defaulting the the start and end duration to 0 when employee tries to enter absence.

Kindly guide how to utilize the runtime passed start and end duration in the Global Absence Type Duration Formula.