If your compass is not MD-corrected (pointing to mag. north), then you'll need to use the little magnetic north (MN) line on the map's north indicator symbol that shows true north (TN), magnetic north (MN), and grid north (GN).
Be aware that little MN line on the symbol is diagramatic only! The number of degrees it states is good (as of the stated date), however
the actual angle depicted is diagramtic only!
According to the USGS pamphlet
Finding Your Way with Map and Compass:
"A diagram in the map margin
will show the difference (declination) at
the center of the map between compass
north (magnetic north indicated by the
MN symbol) and true north (polar north
indicated by the "star" symbol). This diagram
also provides the declination
between true north and the orientation of
the Universal Transverse Mercator (UTM)
grid north (indicated by the GN symbol).
The declination diagram is only representational,
and true values of the angles of
declination should be taken from the
numbers provided rather than from the
directional lines."
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