Hi
I know exactly how you feel! I failed nav first time round due to the cumulative effect of track distances. To be honest, I don't think that the exam is totally fair as you can read the distances half a mile one way or the other. It doesn't make a whole lot of difference in the real world but can spell disaster in the exam! My second attempt was much better and I passed with flying colours so to speak but even then I had problems with the track distance as again I could have read it differently rounding it up or down. I worked backwards on the calculations in the end and got the right answer but of all the exams I wouldn't want to do nav again due to that reason alone. The calcs were fine and had no problems - it was just my interpretation of the measurement despite being totally anal with it and drawing the track line with a permanent fine liner pen.
I remember feeling quite gutted as I have never failed an exam before. If it makes you feel better I went on and got 90% + for all my other exams and ended up with a 95% average pass score. Its just one of those things but next time be very methodical and re-check your answers.