I remember having a very similar issue from the PPL Confuser book and I think I may have got one in the exam. I attributed the error to variability in the computer when calculating the wind correction, rather than measuring the initial angle from the chart.
The exam question I had the biggest risk of losing marks on was an early question getting you to plot the initial angle from the chart and I couldn't find the starting airport. The answer from that question was used for a further 4 or 5 questions. Threw me because I hadn't come across questions using map co-ordinates before, so worth having a wee practice on finding map locations from co-ordinates before the exam. Best of luck mate.