If gross salary fluctuate one may get down from P.tax slab - In that case my P.Tax will be?

If gross salary fluctuate one may get down from P.tax slab. For eg. My gross salary is 18000 so my PTax(West Bengal) is 130 and since I was present for 21 days my gross salary is standing to Rs.12600.In that case my P.Tax will be 110. I just need to know how can i put this in excel formula.Please help me out.

PT is applicable month-wise gross payable, if it get reduced, then PT lower slab will get effective

Dear Friend,
PT deduct on total earning of the employee (Including Overtime,Incentive etc...), Put your formula on Total Earning amount.
Ex :
=IF(TotalEarning<=7500,0,IF(TotalEarning<=10000,17 5,IF(AND(TotalEarning>10000,$F$3="FEBRUARY"),300,2 00)))
Use this formula in your excel and change the figures as per your state slab.