Magento – Coupon wrong Grand Total Caclulation and minimum order value… again

I am not sure if this would help you somehow, but I think that you can calculate with these two files, or modify the functions:

Tax subtotals file calculation is done here:

magento\app\code\core\Mage\Tax\Model\Sales\Total\Quote\Subtotal.php

public function collect(Mage_Sales_Model_Quote_Address $address)
    {

There you have round functions used:

Further magento takes these functions from this file:

\magento\app\code\core\Mage\Tax\Model\Calculation.php

Check these functions:

public function round($price)
    {
        return Mage::app()->getStore()->roundPrice($price);
    }

    /**
     * Round price up
     *
     * @param   float $price
     * @return  float
     */
    public function roundUp($price)
    {
        return ceil($price * 100) / 100;
    }

    /**
     * Round price down
     *
     * @param   float $price
     * @return  float
     */
    public function roundDown($price)
    {
        return floor($price * 100) / 100;
    }

I am not sure if this is the right answer, but hopefully this can get you somewhere. Because magento pricing is quite complex.

This is most likely due to the Tax Origin not being set correctly (it's probably on its default: US). This is different from the Default Country setting, you can find it at System -> Configuration -> [SALES] Tax -> Origin.

Let me know if it works! :)