Should Muslim names really be linked with Christian names ?
The linking of the names does 2 things:
1. It makes the numbering of rulers correct between different cultural-names
2. Characters who change culture get the equivalent name in their new culture
For 1, I don't think that lets say a Scottish duke of Damascus named David would call himself David III because there already were two Muslim Da'ud before him or vice-versa.
For 2, this happens only very rarely and people usually choose a whole different name if they converted, so having this link isn't that important IMHO.