I too use straight-grain binding. I use a formula of 6 times the finished width of the binding + 1/4"; example: 6 x 1/2" = 3 + 1/2" = 3-1/2". I sew the ends of the binding pieces together on an angle to distribute thickness. I then press the binding in half to 1-3/4". Sew the binding at 1/2" around the quilt. The extra 1/2" of width is taken up in the thickness of the sandwich and allows ample to cover your stitch line.
All this to say that my binding is double. If one of your sides measurements is longer than the opposite one, you can take up this difference using a straight-grain binding to make them the same.
I only use bias on curved edges but it is also double.
Suzanne