If you were selecting a navaid to hold at a VOR you would also set the inbound course so the FMGC is not doing anything out of the usual.
For a gross error check go to rose nav and check the opposite of what you have been given.
To be brutally honest if you cannot add/subtract 180 to a number you are probably in the wrong job. God forbid what they would get upto if they had to do manual holds with a crosswind!!