A direction written as “080°” looks precise, yet it leaves an important question unanswered: measured from which north? Before moving a bearing from a publication into a notebook, retain the label that explains its reference.
Keep the reference with the value
NOAA’s explanation of magnetic declination defines the angle between magnetic north and true north and notes that it varies with location and time. The two references should not be treated as interchangeable simply because both use degrees.
For a study exercise, copy a labelled direction exactly as presented, including its reference and source. If the label is missing, mark the entry as incomplete rather than choose the interpretation that seems most familiar.
Do not borrow an adjustment from another example
A worked example in an older book belongs to its stated place, date and assumptions. Its adjustment is not a number to carry into a different harbour. The purpose of this reading exercise is to understand the labels, not to supply an operational navigation procedure.
Our guide to checking a nautical publication's edition is useful beside this habit. For actual navigation, use the appropriate current information, equipment guidance and instruction for the voyage.
Write a note another reader can check
A useful notebook entry records the value, whether it is true or magnetic, the source, and the relevant date or location supplied by that source. Keep your own interpretation on a separate line.
If two references appear to disagree, first check their labels before assuming one contains a numerical error. Sometimes the missing piece is not a more detailed calculation, but the small word that tells you what the calculation is describing.