Robin,
Try
http://www.rya.org.uk/sitecollectiondoc ... ATIONS.pdf
I'd proceed as follows, being a bit on the prudent side:
Tie the mast on the roof rack so that about the same length sticks out over the front and the back.
If the mast sticks out more than a metre at both ends, have it stick out by 1 metre at the front and tie a large bright coloured old rag on the rear end.
If in doubt, tie the large colourful old rag on the rear end anyway.
I have no suggestions on sourcing a bright, preferably fluorescent, old rag......