It's quite correct to use CDMVT, but lets go back to the basic definitions:
Variation is the angular difference between True North and Magnetic North. The sense is defined as West IF magnetic North is west of True, and vice-versa.
Deviation is the angular difference between Compass North and Magnetic North. The sense is defined as West IF Compass North is West of Magnetic North.
To put this in context:
Draw a vertical line on a sheet of paper. This will be True North. Now draw an aircraft pointing left to right. The aircraft will be heading 090T.
Now draw a line 10 degrees to the left of true North. This will be magnetic North. Because the line is to the left of True this will give 10 deg West Variation. The Magnetic heading will now be 090 + 10 = 100M.
Now draw another line to the left of Mag North, again 10 degrees. This will be Compass North. The Compass North is West of Magnetic so the Deviation will be 10 West.
The aircraft has not moved so the compass heading will be 090T + 10W Var + 10 W Dev = 110 Compass.
The way I teach this is to use CDMVT but link it to the Cardinials -
Draw a cross, vertical, and annote the 4 ends with N,S,E and W (I trust you can cope with this one!).
Start in the centre of the cross. If you go right you are going East - so add easterly variation. If going left you are going West, so add Westerly variation.
C D M V T
Mag to True you are going right thus add east Var.
For deviation, which is usually shown as + or -, we normally write from left to right thus if going from left to right you apply the sign as given. If going "the wrong way" right to left you reverse the sign.
C D M V T
If dev is -10 then: Comp - 10 = Mag
Sounds awkward, but in practice its very easy.
By the way the correct answer is 21W!
Mike
Chief Ground Instructor
OAT