Remember the rule, grid to mag, add. Mag to grid, get rid
That would only apply in some area where the variation is west of true north... unless you're talking about adding or subtracting
positive or
negative numbers.
In real life, and as modeled in FS9, the variation can be expressed in terms of "negative" for areas where magnetic North is East of true North (which means your true course heading will be a lower number on the degree scale than your magnetic heading), or "positive" when it is West of true North.
Of course, if you go past 360 degrees while adding or subtracting, that changes, but you have to visualize the circular scale, not a straight line. so, if you've laid out a true course with a heading of, say, 005 degrees true, and the variation is 10 degrees east, your compass heading to follow that course line needs to be 350 magnetic, because you subtracted 10 from 005.
On US charts, magnetic variation lines are labeled "xx degrees E" or "XX degrees W", so one has to figure the add/subtract thing every time.
Fortunately, there's another useful mnemonic for that:
"East is least, West is best".
...!
