Now, your compass deviation, if westerly, has to be added to this averaged variation, and vice versa if easterly (one minus sign).
Ngoku, ukunxaxha kwekhampasi, ukuba iseNtshona, izakudityaniswa kulo mndililili woguqu-guqulo njalo-njalo ukuba ingasempuma (kwenye uthabatha isalathiso).