If my great great grandmother was part of Clan MacGregor can I belong to the Clan aswell even though it's so far back? Not officially belong to the Clan but can I call myself a MacGregor?
Clans imagined as groups of family members are mythical creations mostly started as a PR exercise by Sir Walter Scott and perpetuated today because it sells tartan tat to gullible North Americans.
If you stop to think for a moment it is not even vaguely conceivable that everyone of a single bloodline and surname lived together in one area to the exclusion of others of different bloodlines and surnames. Largely it was people living together taking the name of the leader, at a time when surnames weren't commonly used, but having no blood links to that leader.
Note that the surname McGregor was outlawed from 1603-1775, although it was briefly repealed 1660-1693.
Of course the myth of tartans having specific patterns is the most laughable part of the whole "clan" story, and the advise is you are "entitled" to wear any damned tartan pattern that you like, no one is stopping you and no tartans were ever historically associated with a family that's an invention of two English brothers that purported to be descendants of the old pretender.