From the Visit Scotland website, Barmaid:
The idea of clan and family tartans is thought to be a relatively new invention, following the standardisation of tartans worn by the Highland regiments of the 18th century, and during the Victorian period, when all things Scottish were considered fashionable. Prior to this, it is believed that clansmen were known to wear variety of different setts and the weave of cloth tended to be dictated by the custom of a particular area, rather than by clan or family affiliation.
There's a lot of kidology around tartan and you could spend a long time in conversation with earnest professionals trying to find your answer.
If all else fails, adopt something like this:
http://www.tartanregister.gov.uk/tartanDetails.aspx?ref=10545