Hello there!
It's true, we live in a GPS age, so all those question about TT WCA CH etc. have more practical relevance before the examination table than when flying.
What we usually know from the map is TT from A to B, and what we want to know is CH.
There is a single fomula that takes care of everything. It goes like this:
CH = TT PLUS WCA MINUS VARIATION MINUS DEVIATION.
Do not forget: wca as well as var. and dev. can all have positive or negative values, if the crosswind blows from the left, the wca is negative, and if magnetic north lies left (west) of true north the var. is negative etc...
An example: TT=265, WCA= -20, VAR west 3, DEV east 4.
CH=265 +(-20)-(-3)-(+4)
=265-20+3-4
=244.