How to allocation less amount through assessment Cycle for overhead calculation in process order

Hi

i need your suggestion and input for above type i.e. Assessment cycle for Overhead Calculation (on Quantity bases)

i had book an invoice of  for example Salary  : 5000/-

System will allocate the cost through assessment cycle by using SKF 2:3 (cost centre 1 =2000/ and cost center 2=3000/-)

but client requirement is need to allocate instead of 5000 only he need to allocate in 4000/- (with 2:3) ratio

How can we map this scenario in the System please assist further

Thanks

Trinath