This quote is followed by a table, shown right, of possible unfinished block sizes. [Please excuse the missing horizontal lines.]
Of course, the "Unfinished Block Width" is the size of the finished block plus 1/2".
The "Sewing Line" is the size of a leg of the corner triangle as measured from the corner to where the sewing is to start on the diagonal. In other words, using the "Flip & Sew" technique for corner triangles, it is the size of the corner square.
As a result, if you are interested in getting a perfect looking snowball quilt, invest in the "Mastering Perfect Corners" combo which has the Perfect Corner Ruler as well as the "Drawing The Line On Accuracy" for $14.95 US.
I must apologize. I did not intend to use this Q&A for sales. However, I know that I could not do a snowball without the Perfect Corner Ruler.