Conveyance Exemption Error

Hi Experts ,

For one of my client pay structure has been changed ,we have used all the new wage types based on the current requirement ,now problem is previously they are not using standard process of conveyance exemption ,now they want the standard conveyance exemption for the employees ,so based on that I have maintained all the tables mapped the wage type with the SCNV ,but the problem is

if employee joined in the August conveyance exemption /4E3 is = 1600*9 = 14400 which is correct but in the September month the /4E3 is - 12800,in October it is 11200 as it is decreasing when payroll run goes for the next month ,in March the /4E3 is displaying only 1600 .

I am unable to find where the exact problem ,please help me as it is very urgent .

Thanks,

Anusha .