I'm kinda shocked to find that there is not one picture of a painted tartan on this site!
What I have done in the past is to get some good photo/drawing references in poses the same as the figure I'm painting, then follow them as a guide. Sometimes you need several images for one figure, as they are never exact.
I'm SURE there are pictures of Black Watch around, so download a bunch, match up the poses, and you have a least a partial guide. My last (only?) tartan was a round figure, so not much help there. Before that, I did a bunch of toy soldiers, but that's hardly a comparable effort.
Hope this helps.