Two possibilities: either use 1 strand of floss (which will look thin when stitching over two; imagine using 1 strand when stitching on 14 ct, see what I mean?) or more likely, use 1 strand but stitch over one. Are you by any chance starting a HAED d...
Great on silk, rayons, polyester, lycra, acrylic, etc. If you want to iron COTTON AND LINEN, you best be ready to go over the fabric at least 3-5 times to whip the fabric into submission! Hope this helps.
Try this: line the top with iron-on or sew-in interfacing to turn it non-stretchy. After that, you can tell how wide the bottom part will need to be. .