What's the difference between true, magnetic, and compass bearing?
True bearing (Β°T) is measured clockwise from geographicnorth. It's what a great-circle calculation produces and what appears on a paper chart's compass rose ring labelled "true".
Magnetic bearing (Β°M) is measured from magnetic north. The difference between the two, magnetic declination (or variation), varies by position and changes slowly over time. East declination is added when converting magnetic β true, so to go true β magnetic you subtract it.
Compass bearing (Β°C)is what your boat's compass actually reads. It differs from the magnetic bearing by deviation, a per-vessel error caused by nearby ferrous metal and electrical fields. Deviation is heading-dependent and lives on the boat's deviation card.
The "Auto estimate" declination button uses a centered-dipole approximation β it's only a rough hint. For real navigation, use a current WMM/IGRF value (e.g. NOAA's online calculator).