Limit the amount of absence hours deducted from qualification plan entitlement

Client requirement: Employees are eligible for 15 weeks of parental leave and upto 12 weeks of Short-term disability. Parental leave and STD plans are combined for processing. When an employee requests for parental leave total duration is capped to 15 weeks. However, only first 8 weeks of leave must be deducted from STD plan entitlement if enough balance is available and remaining from parental leave. Also, FMLA plan runs in concurrent with both the plans for entire 15 weeks duration.

Both STD and parental leave plans are configured as qualification plans and we have combined STD (priority 1) and parental leave (priority 2) plans using a same plan type without enabling plan level concurrency. Issue with this is, system will deduct upto 12 weeks of entitlement from STD when enough entitlement is available and the left over duration from parental leave plan. We are exploring the option of using conversion formula at plan or absence type level, can someone please let us know if it is feasible to achieve this requirement using a conversion formula or if there is any