Decide (and use consistently) a column break threshold

[barrucadu]

It's not very aesthetically pleasing to have a new chapter start at the bottom of a column, but how little text is too little to allow?

A \columnbreak must be used if there's only one or two lines, as the chapter number is three lines tall and it really looks bad if it dips into the footer. Four lines looks good. But what about three lines? Should that be broken? At the moment I'm doing this inconsistently.