I needed to trim the extra 1/4" I had added to each side of the Harlequin block before I could take off the papers. The back-stop ruler was taped down such that the 15" square ruler would cut on the 25" line on the cutting mat.
I aligned the block to overlap the 25" line on the cutting mat and laid the square ruler in place, making sure the cutting line on the paper was right at the edge of the ruler.
Having the back-stop ruler in place really made all this 1/4" trimming simple; however, having the extra allowance on the outside of this block turned out to be unnecessary. I do not think I will ever add 1/4" again to a large paper-pieced block again. Live and learn.
Susan in Texas