Many thanks for your replies. Me thinks i may be reading a bit tooooo much into this section. I can calculate the error at the destination, and i understand how it works. The Schuler bit just freaked me out a bit.
Spitfie: Myintention was not to make you scream and run!!! Its hard going, but appears to me to be the huge volume of information, and not the difficulty. It is hard, hard work though. Rewarding though when it starts coming together !!