Jim, All of the above theory is true(pun), but the compass rose you are referring to is indicating 13 degrees (12.5) WEST variation. Keep it simple to start out. EAST is least(-)WEST is best(+), keep this in mind when working from True to Mag. Use the TVMDC Sailaway was talking about above. Write it vertically with T at the top and fill in what you know, add or subtract as necessary to fill in the blanks. In the compass rose you linked to it shows the variation of 13W. So if you have plotted a True course on your chart in order to figure out your magnetic course(forget Deviation for now) you would ADD 13 degrees from the True course to get your Mag. heading(000T + 13W = 013M. Are you confused yet?
D 4E Deviation follows same rules as Variation
C 009 Compass course to steer or psc (per steering compass)
If you always write out the TVMDC you can't go wrong if you add all west and subtract all east variation and deviation.
SV Laurie Anne
1988 Brewer 40 Pilothouse