I think putting the focus on the measurement of the seam allowance is what confuses people. It doesn't matter what the measurement of the seam allowance is. What matters is the size of the finished block. The question to ask is, 'What seam allowance is going to result in the block being the size it is supposed to be?"
You have to experiment to see where to set it on a particular machine. Someone recently posted a video that suggested cutting a nine inch strip of fabric, and then cutting nine 1 1/2" strips of fabric, sewing the 1 1/2" strips of fabric together, comparing that length to the 9" strip, and adjusting the seam allowance until the sewn strip matches the 9" strip. Whatever that turns out to be, that's your 'scant 1/4" seam allowance'.