The link below covers the subject of direct reading magnetic compasses and compass errors.
But take care in using it.....it was written in Australia so it is for the southern hemisphere. The effects are all reversed in the northern hemisphere.
CompassesThe north magnetic pole is situated in Northern Canada, and the south magnetic pole in Victorialand, in Antarctica. Direct Reading Compass. Fig 1. Fig 2. avfacts.com.au/sample/navinst2a.PDF - related