Advice for sewing large areas of tartan?

[ardchoille]

I'm working with 13 ounce tartan in MacGregor modern and need some advice for sewing in the undercheck (red) region. I know I can baste/pin before sewing but that still seems to leave a fold of fabric and I don't know where in that fold to sew in order to end up with a straight seam. What would be the dangers of using tailor's chalk on wool tartan cloth? Does chalk brush out as easily in tartan as it does in cotton twill using a toothbrush? If I can do this without any problems to the cloth I can end up with a straight line to sew (following the chalk line) and end up with a straight seam for my box pleats. How do kiltmakers obtain such a straight seem in their pleats?

The area I am talking about is the yellow box in the lower right corner of this image: