[QUOTE=Barb T.;902018]So, it's not a question of either twill line or where the mill decides to start the tartan when winding the warp.QUOTE]

I would have to disagree here. It's all to do with maths - sett size, number of repeats or runs and the width of the warp. One can play endlessly with the count to make it fit but that generally requires a serious compromise on the overall proportions of the sett and often has the effect of making the finer stripes more prominent as it's easier to reduce the ground colours. A fiddle in the count, especially asymmetric ones, is sometimes required with double width cloth but for single width cloth there is no excuse for a light colour on the warp.