Accrual Logic for employees working on different Shift Pattern

Summary

Should we accrue absence based on weekly FTE or based on shift duration

Content

We have people who work on various elapsed shifts and the duration may or may not be same each day. But their weekly FTE remains same. The accrual for these employees would be front loaded.

Should we write a fast formula to give the absence entitlement based on their FTE or should we check both FTE and the duration for each day? 

If we give the accrual based on FTE then on days where they work more hors, then they would have to enter enter absence for more hours than they accrued and maybe short of hours compared to another employee who works on same duration for each week.