Teroc - You might also have a look at Martyn Smith's little book "Diversion Planning", which is very good for showing you how to do most of the routine navigational problems in your head. There are only 21 pages in it (single-sided pages, at that) and it's very readable. Before you ask, no my name isn't Martyn Smith! I think it's one of the best books I've seen on the subject, £8.50 and I think you'll get it from Transair.