When you compare the Grid Track and the True Track, if the True Track is a larger number, then Chart Convergence is West. If True is smaller, Chart Convergence is East.
So Convergence West - True Best
Convergence East - True Least
With this information, you can work out whether the longitude at B degrees West (in your example) is East or West of the Datum. You need to know this, otherwise you may apply the change of longitude the wrong way.
If convergence is west, B will be west of the datum in the southern hemisphere and East of the datum in the northern hemisphere.
If convergence is east, B will be east of the datum in the southern hemisphere and West of the datum in the northern hemisphere.
Well, nobody said it was supposed to be easy!!!