I found the Nav exam to be a bit frustrating. The answers require you to be very accurate with your calculations, and if you have a bit of a cheap 'whiz wheel' then you might have problems. I borrowed an 'ATPL style' whiz wheel for the exam because it helped accuracy. In real life however, mine works perfectly well with its 2 degree resolution; seeing as the aircraft I fly has a DI with a resolution of 5 degrees.