Originally Posted by
Peckish
A quarter-inch seam is where ever you have to place the fabric to get a true quarter-inch seam.
That's not very helpful, I know.
You're going to need to sew some scraps, then measure the finished patch to get the correct finished size before you can know where to place the edge of the fabric with regards to your foot.
I agree with Peckish. Don't mess with measuring the seam, measure the finished blocks on your sample to see how accurate you are. I used to measure the seam and it was never accurate. Once I started making a couple sample blocks and measuring those instead, my blocks were much more accurate