I decided to do the block of the month for May, piecing a single block gives me something to sew without a big commitment. The unfinished block size is 13", which is a little odd, but okay... Here it is laid out on my piecing board. I cut carefully. I checked the quarter inch seam on my machine and stuck a piece of mole foam on the machine bed as a guide, I sewed what looked like a thread breadth shy of where I normally sew.
The results: Points all match, there's a healthy quarter inch around the outside for seam allowance, block looks good. It measures 12 7/8"
Hiding somewhere in my sewing room is the elusive 1/8".