I don't belive I'd feel comfortable wearing the tartan of a Clan without some sort of claim to that right.

Lineage is not my aim. I doubt I could easily prove lineage back to Lady Jane. Particularily with a name like Grey (Gray). It is a matter more in line with the guidance given by most, including the Standing Council of Clan Chiefs (I believe). If your family name is the name of a Clan or Sept, you can comfortably expect to wear the tartan of the that Clan.

Does the right of a Clan member to wear an entitled tartan end at some point. Is there something I'm missing?