tax calculation customization

[nadeesha@cnglobal.com.au]

Hi,
I have product including tax in my shopping cart. Then the sub total is also including the tax.
In vertuemart, total tax is calculating based on sub total and tax is calculated as subtotal* mytax_rate.
Then the tax calculation is wrong as it calculate tax from the sub total that already includes tax.

I want to customize it to calculate the tax based on the price excluding tax.

My customer provides prices with tax.
can I use the calc_order_tax method to get this done??? if so, where to customize that method?

Please help!!!!!!!!!!!!!!!!!!!!
Thanks