Change coupon discount calculation

Question

I'm new in Magento, and if someone can bring me some light is there a way to overwrite the calculation method when discount is applyed?

Mage version 1.9.4.

The problem is for eaxmple how mage is calculating the product price excl. VAT, and apply VAT after discount

//We have this rounding in Mage.
function roundPrice($price){
    return round($price, 2);
}

$productPrice = 61.67;
$qty = 2;
$discount = 25; //25%

$subTotal = roundPrice($productPrice * $qty);

$discount = roundPrice($productPrice - ($productPrice / 100) * $discount);

var_dump($subTotal - 30.84 (Fixed discount amount)); // Result float(92.5)

var_dump($discount); // Result float(92.51) I need to get this one cent instead of mage default calculation from convert percent into fixed