The nature of the PPL navigation exam is such that it is very easy to know the correct techniques and use of the computer, but to have introduced cumulative errors from track measurement, magnetic variation interpolation and other sources. Those half degree errors can soon add up to an answer just outside the correct one....particularly when using a rattly old whizz-wheel.
But always plot as accurately as you possibly can!
Multi-guess 'digital' answers are wholly inappropriate for results obtained using 'analogue' methods; personally I think that a 2 or 3 degree tolerance value should be allowable for answers in the PPL exam - if the CAA want more accurate answers then you should be allowed to use an electronic navigation computer. It's only about 40 years since they first appeared on the market, but there are still those who think that an astrolabe, quadrant staff and lodestone are essential tools for PPL navigation...
Incidentally, you ARE allowed to use an electronic calculator for arithmetical purposes only, provided that it doesn't have any 'navigation functions'.