18 January 2011
First calculate the annual estimate of Taxable salary & taxes thereon on the basis of salary breakup & investment / tax declaration submitted by the employee.
Then just divide the estimated tax by 12.
That will be the monthly TDS on the salary.
In the last month (Normally March), recalculate total taxes on the basis of actual receipts, actual investment proofs & total TDS paid till date & deduct the balance TDS in the last month.