You have to learn how to use that stupid circular slide rule.
The CAA exams are rigged to catch out common slide rule mistakes - even if it is just 1 degree out. The fact that not even a £40,000 autopilot can fly to 1 degree is irrelevant
Also make sure you have the same VFR chart which the exam assumes. The magnetic headings etc have changed over the years and a current chart will yield the wrong answers.