There is no way to directly adjust the cart total. But it seems the cart total should be working correctly. I did an example by adding an item which is $10 including tax. and then i adjusted the calculation to be [Price] * [Quantity] * (1/1.1) and added the tax rule. And when i add to cart I'm getting sub total of 9.09 and a tax of .91 and a grand total of $10. Is that the result you are looking for.