What course should be taken to make estimated calculations?
I would calculate that leg in two segments.  From GIJ to BENJO flying a course of 082 magnetic (50 NM), thence from BENJO to CRL on a magnetic course 087 (79 NM).

In addition, the magnetic variation is only applicable to hand plotting an arbitrary course using the map's orientation as a reference; IFR charts list true magnetic courses on their airways for ease of reference.

After that, you just need the winds aloft and you're off to the races!