If you have ever read a quilting book by Mary Ellen Hopkins, she states that when you are sewing blocks together for a quilt, use the same machine for the entire quilt with your own "personal, private, measurement - PPM) and it has worked for me. I do have a 1/4" foot which helps me keep my seams consistent. Hope that helps.

I made this quilt also. I found that when sewing them together, & one side being a little off, all I had to do was pin, only at the seams, & at each ends. That will show you what part is a hair off. The side that is a touch longer put down & the touch shorter on the top as you sew. The feed dogs will take up the extra when sewing together. When doing this, you can also give it a little stretch as it will not affect the looks of the completed block. Hope this helps along with the other hints given you.

Creative Grids made the 20-1/2 square ruler I have. Bought it at Country Stitches in Jackson, MI or their East Lansing, MI store.