The remaining deviation must be corrected for, and this is the main job of the professional compass adjuster.. At first this seems a simple process, just add equal and opposite magnetism near the compass to cancel out the offending magnetism.
(For those of you who want to know more, the effect of one magnet on another is inversely proportional to the square of the distance they are moved away from each other. In other words, moving one magnet away from another magnet 3 " reduces the effect they have on each other by a factor of 9, the square of 3. i.e.: Putting very small magnets close to your compass will cancel out the big magnet on your boat, the engine.)
In theory, error is always equal and opposite on the cardinal headings, (Negative East error equals Positive West error), fixing East should fix West and fixing South should fix North. But in reality, theory is not what we find in the real world. The art of compass adjusting is dealing with removing as much deviation as is possible on all headings, which almost always will leave you with some remaining compass error. You will use the deviation table provided by your compass adjuster to correct the difference between what your compass reads and your intended heading.
Every navigator should memorize the following list in order from top to bottom:
Going down the list from True to Compass, ADD WEST Error and SUBTRACT EAST error.
Going up the list from Compass to True you must SUBTRACT WEST error and ADD EAST error.
Of course, you get the value for Variation from your chart on the compass rose and you get the value for Deviation from your Deviation Card.
As most electronics, (Plotters, GPS etc.), usually display TRUE headings you must always do the above math to correct for your compass error.