Advance tax calculation

At a Point of Calculating Advance Tax, Say 15th Sep., We have to Proceed with the Income so far earned (From Turnover Achieved, G.P. Ratio and the Expenditure so far made and also taking into account the Annual Charges like Depreciation, Interest on Borrowings Etc). Annualise the Same to Whole Year, arrive at the Tax Payable. And Then Estimate Your Advance Tax for Sep Due