Deviation is by definition a correction applied to Compass to yield magnetic.
However as pilots these days the most common task is to calculate the required compass heading to fly to achieve a desired magnetic heading.
BUT it hasn't always been that way. These days we generally already know where we are. GPS or radio bearing from VORs don't require us to correct for deviation. But if you imagine having to fix your position by taking bearings on things, perhaps by ADF or RADAR or even visually (after all, a lot of this stuff has it's origins in Sea naVigation).
If you spent enough time position fixing like that you would find your most common task became converting compass into magnetic rather than vice versa. At which point:
- defining deviation as the correction to be added to compass to yield magnetic,
And
- defining west deviation as negative and east as positive.
(and along similar lines, variation as the value added to magnetic to calculate true)
Makes it quick and easy to do the arithmetic. No need for all the 'helpful' memory joggers that people have subsequently come up with. When I say helpful I am being sarcastic, since I'm from the UK and we have westerly deviation, the 'west is best' saying tends to make people think that by definition westerly deviation or variations are positive when in fact the reverse is the case.
This is very typical of aviation where we have an unfortunate tendency to avoid anything perceived as difficult. I say 'perceived' because adding a negative number is in the primary school maths syllabus. it's hardly degree level. So instead of teaching something that is actually pretty simple anyway, it is dumbed down even further, in the pursuit of 'flight without formulae', such that there is no longer understanding and instead rote knowledge.
C + D = M
M + V = T
East positive, West negative (for D and V)
honestly, isn't that simpler than 4 rhymes?