Pro-Rata for Fixed Shift Leavers

We currently have a pay code for Fixed Shift which is based on a %age of Basic Pay. EG 20% of Basic Salary. The salary is able to be Pro-rata when someone leaves but the fixed shift does not then pick up on the Pro-rata, amount it continues to calculate 20% of the Basic Salary which is not correct. There is a work around by adding another pay code and manually entering the figure but this is not ideal.