how to do the calculation

Question

i design application using c sharp.

in that i have four column as follows.

Diesel cost, Rental cost ,Diesel service, Service Tax.

Diesel Cost      230291

Rental Cost      231270

Diesel ser         19767

Ser Tax            30124(231270 + 19767 * 12/100).

i am doing service tax calculation at that time service tax is 12 percentage.suppose in next year service tax percentage is 12.36 how to do that calculation.please help me.

Rgds,

Narasiman P