You can make an override for the productdetails layout and calculate the percentage there.
It's just a simple calculation like this:
if ($product->prices['basePriceWithTax'] != 0) { // with tax
$percentDiscount = round($product->prices['discountAmount'] / $product->prices['basePriceWithTax'] * 100, 0);
} elseif ($product->prices['basePrice'] != 0) { // without tax
$percentDiscount = round($product->prices['discountAmount'] / $product->prices['basePrice'] * 100, 0);
}