I just wanted to add one more thought about magnetic deviation before this thread becomes inactive:

You can determine your current local deviation by locating the North Star (Polaris) and determining its magnetic bearing. I won't go into how to locate Polaris because this is well covered elsewhere. Making a compass sighting on something high above the horizon is difficult, but can be eased with a plumb line.

Polaris is always within 1 degree of true north and can be used from much of the northern hemisphere. This would give you a value for deviation if you have no access to charts, etc. or would allow you to confirm the value you got from a (possibly outdated) chart. It could also resolve any lingering doubt or confusion about whether to add or subtract the deviation, since you are directly observing true north.