The idea of the rulers document is that you pick a paper size from your
printer driver which has no margins (typically it is called 'A4 Paper
Size'), load the rulers document and print it. From how much of the rulers
appears on paper you can work out what the correct margins for your printer
are.

The motivation for this is that all too often Acorn printer drivers have
much larger margins than needed, which can lead to material being clipped
unecessarily.

The document and the idea were provided by Tony van der Hoff.