This means the adding or subtracting of the variation, as given on the chart, and then doing the same with this result and the compass error (deviation) as on your recently constructed deviation curve.
Itheth'ukuthi ulwandiso okanye uthabatho boguqu-guquko njengoko selubhaliwe kwesisalathiso sephepha senza into enye ngezi ziphumo ize - impazamo yekhampasi (ukunxaxwa) njengokwenza kokunxaxha kugoba.