What waistcoat to wear with Stewart Black tartan?

[Nathan]

I think the charcoal waistcoat with the shawl lapels in your OP will work great with that tartan, provided that it's not too long as mentioned above. I've had the bottom of a waistcoat shorted and it went from a pointed bottom to a straight bottom. I really like the result.

People on this board might erroneously conclude that because I tend to wear my kilt traditionally most of the time, I don't approve of contemporary approaches. That's not the case. I just think some modern twists work very well while other, "I'll wear whatever I want" ideas look ridiculous. When people ask for my opinion, I'll share it.

If you're going to break the conventions, it can't be random. It needs to do it with a bit of an artist's hand.

I think you have a good sense of personal style and would encourage you to trust your own instincts. They seem to be working well for you.