I don't have a special ruler for cutting bias either but if you start at one corner and cut a 45 degree angle and then use that as your ruler, I would assume that would do it. Also I have a special "pearl" foot I used to use to sew the piping into its casing and then onto whatever it is I want to attach it to. But now I've gone to using my zipper foot as I have better luck getting really close to the edge. At least this foot works for me. May not work for you.