NavList:

Thanks David. Coincidentally I also have one of those Stationery Office rulers. Must have taken it home accidentally. I was contemplating cutting it down the middle but I've actually knocked one up on a spreadsheet. Surprisingly quick and easy to do and only took a few goes to get the scale. I'm going to print it onto transparent vinyl and laminate it with some perspex. It was so easy I'm going to make some longitude scales for my position too. It's not going to solve how to use the device though. I'll have to experiment. Any further advice appreciated.