Umm... saying you want to steer a course of -13˚ doesn't make sense...you've actually got it backwards. The magnetic variation is
13˚ WEST.
For example, if you want to sail a course of 0˚ true, then you would steer a course of 13˚ magnetic. If you wanted to sail a course of 45˚ true, then you would steer a course of 58˚ magnetic.
Basically, the easiest way to find the variation is to look at the compass rose and line up on True North, and what ever is lined up with it is the variation—in this case 13˚ West, since the 0˚M is 13˚ west of 0˚ True. Variation and deviation have both a magnitude and a direction.
Now the discussion above assumes that the compass doesn't have any deviation, which is not usually the case.
To go from Compass to Magnetic to True, you can remember the mnemonic "
Can
Dead
Men
Vote
Twice". It stands for Compass, Deviation, Magnetic, Variation, True.
This means to go from Compass to Magnetic, you add the Deviation, to go from Magnetic to True, you add the variation.
BTW, when you are correcting a heading or course—ie going from Compass to True— you add East... When uncorrecting—going from True to Compass—you subtract East.
ed: corrected variation from 13˚ East to 13˚ West.
My bad, thanks Sailaway...