Was toying with the idea of basing it on a minimum estimate to save the palaver of bills etc. e.g. My electricity bill is normally €80 per two months, so an average of €480, so was going to go on the basis of €400 euro - 10% = €40, and then at the marginal rate it is €16 back. I know I am potentially losing out on say and extra 3.20 but I'm too lazy to do the exact calculations! 
