I can only think that the + and - thing is mistakenly taken from the common ADF techniques that are taught and then applied inappropriately to deviations of the compass. You cannot calculate deviation of the compass because the errors will only be discovered by undertaking the 'swing'. Each installation will be different. The swing will not be consistent around the clock but must be able to be countered with adjustable magnets that are built into the compass and this is not predictable prior to the swing.