If you are challenged with numbers, go simple and not get frustrated! Just add a strip. But add it in a unique way, not just a piece down the middle or a border. I always add strips, pieces, blocks, something to my quilt backs just because I like the design elements. One a my favorite ways to make a square bigger (if fabric is not a one way design) is to cut it lengthwise at one third width. Add the strip size needed. The cut the piece in half and flip it around. Add another same size strip and sew together. Now your finished piece has 4 pieces (top part is 2/3 and 1/3, bottom half is 1/3 and 2/3). Hope you can visualize that. It is an easy way to make a piece a lot bigger without looking like you ran out of fabric!