If I'm understanding your question, I ran into this problem recently while making HST that were only 1 and 1/2 inch square so the slightest move caused a problem. I found that when making the diagonal cut if I used the middle of the edge of the ruler on the line rather than placing the corner of the ruler at the beginning or end of the cut, it did not move at the end. I hope you can figure this out, probably clear as mud! As for chain piecing, I have no suggestions for that.