Someone told me to use a "scant" 1/4 inch seam. Sew a 1/4" seam, then move your needle to the right one notch. This should be a scant. If you are making blocks, you can then square them. I do know what you mean. After I cut one length, I check the remaining material with my ruler to make sure it is square, if not, square it again before you cut another length. I hope that this makes sense. Rulers can move on you when you are cutting. Is the table you are cutting on, flat? I noticed that my cutting board does have a dip in it.