Fuel as a % of Total shipment cost

Hi All,

This is our current business logic :
Total Rate - ZAR10 000
Fuel Split - 20%

To us this means that 20% of the total rate is allocated for fuel,
therefore base = ZAR8000 and fuel = ZAR2000

At the end of a month we receive the % the fuel price has moved and to accommodate for this we need to increase the fuel portion of the rate and recalculate the fuel split as it will now be different

Please help with this setup ?