[fer@fer]

You better find a good book, and of possible a teacher too.

A simple approach is in Celestial Navigation for Yachtsmen by Mary Blewitt, Thomas Bergel

http://www.amazon.com/exec/obidos/tg/detail/-/0070059284/102-8364544-5309709?v=glance

You might also be willing to reach one of the best pages on this subject

http://home.t-online.de/home/h.umland/index.htm

And also purchase or download bowditch''s practical navigator http://pollux.nss.nima.mil/pubs/pubs_j_apn_sections.html?rid=187

As to accuracy, a 4 second error in your watch, represents 1 n mile.

Depending on the approch, 2/5 mile error is plausible. In my case is standard.