So far, all bearings we've used are based on magnetic north. Normally, that's all a letterboxer will deal with...but on rare occasions, someone likes to trick you and give bearings based on true north. It would be nice if magnetic north was the same as true north, but it isn't. A compass will point to the magnetic North Pole, located about 1,400 miles south of the true North Pole, off the northern coast of Canada near Bathurst Island. And that means a headache for you if someone decides to use true north.
The angle between true north and the direction the compass needle points is called the declination. It would be nice if the declination was the same everywhere and you could convert from one to the other with a formula—like you can do with Celsius and Fahrenheit—but it doesn't work that way. Declination varies dramatically all over the planet—as much as 50 degrees just within the United States—and the only way to find the correct declination for a given area is by looking it up. The few letterbox clues that use degrees based on true north usually include the declination to make things easier for you, but if they don't, you'll have to find the decliation on your own.
There are several sources one can use to find the declination for a given area. A good topographic map will show the declination for the area it covers, as will many trail or nature guides. They do go out of date, however—magnetic declination is an annoying moving target! It's not a fast moving target, but if your topo map or guidebook is a couple of decades old, the declination could have changed significantly. Try using the Magnetic Declination website for up-to-date declinations.
Okay, you've finally found the declination for your area. How do you use this number to convert from degrees based on true north to degrees based on magnetic north? By adding the declination to true north. Let's use an example to make it clear. Let's say you are in an area where the declination is 5°W (i.e. -5°). What this means is that your magnetic compass is pointing 5 degrees west of true north. All of your magnetic bearings are five degrees smaller than the equivalent true north bearings. If you point your compass to zero degrees, then true north is actually five degrees east (+5°). Now if your clues says to walk at 20° of true north, add -5 to get a magnetic bearing of 15°—which points to 20° true north.
Another example: Let's say you are in an area where the declination is 12°E (i.e. +12°). You want to follow 305° from true north. Set your compass to 317° (305 + 12) and your off!
What if you're creating a clue and want to make it difficult for others by giving clues using degrees based on true north? You'll do the opposite as before: Add the declination to magnetic north.