Adding or subtracting variation when going from map to compass and v.v. depends on whether the magnetic pole is east or west of you.
You also need to know whether the magnetic variation shown, or given, on the map is from grid north or true north. And you also need to know whether the "north" on the map is true north or grid north. I agree that maps often state all these facts but there are occasions when you get magnetic variation from one source and the "north" (true or grid) from the map - a different source. Also some maps show, i.e. are gridded, magnetic north (which varies over time) and you may want to get back to true, or to a different grid north to correlate with a different map. It's also worth mentioning that not all maps are "north" up (he said remembering being caught out by this recently!).
The rule I use, going from true, or grid, bearing/course on map or chart to compass is "West Best (add variation), East Least (subtract variation)". A simple rule which can be readily modified to go the other way, and when other facts are taken into consideration. Variation is an important topic: whilst relatively small in the UK, in some parts of the world it is well in excess of twenty degrees - which makes a massive difference if you get it the wrong way round (or don't allow for it!).
Just to complicate magnetism and compasses a little further, if you're navigating at sea there's a further correction to apply: deviation. This is different for each vessel, can vary according to where the gear is stowed on the vessel - and varies according to the heading of the vessel. (Amazing we make landfall at all, really!)
As an earlier poster said, it is not a simple topic and a full understanding of the implications is necessary to teach to others. If you are teaching nav "global" principles should be taught, rather than what you do in the U.K.