Rule of thump

A misprint I came across in a report I'm editing today, about banana cultivation in atoll islands. It's talking about how often banana plants should be fed with organic matter and suggests once a month as a rule of thump. Maybe I'm tired and bored (OK, I am tired and bored), but this has tickled me no end. It's like a homelier more approximate version of the law of the jungle. The rule of thump. Let's see if we can introduce into everyday speech. We could start with political discourse. As in: Gordon Brown KO'd by rule of thump.