My Dresden Plate quilt has 8" blocks. The plate is 7" across. I wish I'd made the back pieces larger. I did a tutorial on the DP --- Here's the measurements for the blades.
3 1/2" across the top
1" across the bottom
total length of this ruler is 10" -- for the 3 1/2" blade (7" plate), you make the total length 3 1/2" - Cut your template 10" and mark it every 1/2" inch across. Then lay the bottom of the fabric at the 3 1/2" mark.