Play testing will help determine if the main mast is the best point of reference, but it seems a logical choice. Let's start with it and see what happens.

Die charts could be set up based on ships' bearing, and possibly some consideration of distance (at least with standard and advanced rules), e.g. within two rulers or some such thing. Unlike WGF in which speed is basically set, SGN allows for different sail settings resulting in different arrow lengths, hence the distance consideration.