Well since I’m new here and I’m obviously not more than a novice at sewing my 15-91 has a presser foot that is fatter on one side than the inboard side. I keep my fabric along the inboard little side of the presser foot. I measured the resulting seam allowance with a fabric tape and I can see it at a 1/4 inch. In reality I believe it’s about.210 of an inch. That’s more than 3/16 and less than 1/4 inch. I’m a helicopter mechanic I have to build by the instructions or somebody could die.
When I get done with all of my blocks I plan to lay a card board template and rotary cut all blocks to the same size. At some point I will basically have a blanket. I’m not wrapped up in perfection because it’s my first quilt. I totally understand the pursuit of perfection but unless you are competitively sewing relax, take a deep breath, say woosha. Always room to fudge when you are making something from scratch. Do it because you enjoy it don’t make it work.