Just about any criteria I could think of for wearing the clergy tartan would be artificial and/or exclude people who would have a legitimate reason for wearing it.

For instance, there are people serving God as full-time ministers and in other roles who are not ordained. And there are ordained pastors and missionaries who work another job full-time.

Speaking as someone who is ordained and serving full-time, and who owns a wears a kilt in the clergy tartan, I'm glad there's nobody going around and policing who is allowed and who is not allowed to wear the clergy tartan.

I'd say, as with any tartan, if you feel you are entitled to it (and there are no set criteria), wear it respectfully, and feel comfortable talking with other folks about why you wear it, then go for it.

If you're doing all that and someone else had a problem with it, it's their problem. It sounds like the OP feels he is not entitled to wear it. That's his choice, and I appreciate the respect he shows for the tartan and what it stands for.