Hi,
The calculations are not off. They are correct. If you do all the calculations from the prices without taxes without rounding anything and that you only round the prices for the display you will see that the result is correct. If you do the sum of the elements displayed, it's possible that you see a discrepancy but that's normal since in that case you're doing the calculations with the rounding being done between each calculation instead of only at the end. If you look at your supermarket bills, the calculations are also usually done like that and in some cases, you can see a similar discrepancy in some cases between what you can calculate with the prices displayed on the receipt and the total amount paid.
Similarly, if you turn on that option and that you round all the prices between each calculation, you'll also see that the result is correct.
So there is no other way to change the calculations.