Flexibility in buying game time

Hi! I have an idea. I currently have 5.95 euro on my account so that is not enough to buy 30 days. I did some math and i turns out 1 day of game time is 0,433 euro. After even more math I discovered that 5.95 euro is enough to cover 13 days of game time which costs 5.629 euro. How great would be this level of flexibility in buying game time! Blizzard think it through please