allowance need to prorate from normal to shift, shift amount need to get full amount from shift nrmal

Hi,

in my client shift allowance 300 per month

1) in a month employee  changed from shift to normal  (ex:first 10 days shift remaining days normal ) he need to get full amount (300)

2) in a month employee  changed from normal to shift   (ex:first 10 days normal remaining days shift  ) he need to get should be prorate amount  (200)

how to reach my client requirement  please help me

Thanks&regards

Raamarao