I updated ot_coupon.php file. Calculations for both 'Discount coupon' and 'Credit note' are supported for different type of coupons.
'Discount coupon' are discounts applied on customer order, which mean they are calculated on net prices.
'Credit note' are generally used for refunds and are calculated on products or order gross price.
'Penny loss' problem appeared but was solved by applying a rounding on coupon value just before it is included in order total.
There are still lots of testing/fixing with discount per item option, multiple tax class items in cart, coupon validation in general...