How to calculate Tax Amount in pre sales invoice.

I knew that "Amount Including VAT" field in Sales line gets updated when we release the order. we can use that field to calculate total tax amount of the invoice. but before releasing we can able view the total tax amount in statistics (F9). i need to show that total sales tax amount in sales invoice form with out releasing the sales invoice. can any one suggest me some code to show the tax amount in sales invoice form.