To your credit, our approach never worked :)

I totally agree that you'd need to find a reasonable average weekly cost because costs and timing would vary. In my mind, this means you could find a reasonable average weekly cost that you often go under, and seldom go over. But, it just never happened for us. In principle we could have just kept raising the price ceiling, but eventually that becomes meaningless in the context of a budget. To me, at least, it felt just like calories; what could have been a pretty easy math problem was defeated by human psychology.