The rule is to allow westerly deviation to the left to get the correct magnetic course, and easterly deviation to the right to get the correct magnetic course.
To find out the error of the compass in order to steer a true course, the sum of the deviation and the variation when both are of the same name, and their difference when they have different names, must be ascertained. For instance, deviation 20° West and variation 25° West, would give an error of compass 45° West, which should be applied to the left.
If the deviation was 20° East and the variation 10° West, the difference between them would be 10° East, which compass error should be applied to the right to steer a true course.
In order to find the compass course or course to steer, proceed as follows, the true course being North 40° East, the variation being 38° West and the deviation 18° East:
Variation, 38° W., being of contrary names, take their difference.
Deviation, 18° E.
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Correction, 20°, apply to the right, being westerly.
True course N.40° E.
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