I was watching YouTube yesterday on getting a perfect 1/4 seam allowance and getting a giggle out of all the ways we make this work. Tape. Moleskin. Fancy gadgets and others.
Here's my secret and it works every time: My acyrlic ruler and a mechanical pencil.
Your ruler dictates your 1/4 seam allowance. The machine and it's feet are callibrated to fit the measurements that the manufacturer sets, not always to your specific ruler. All rulers are not created equal which also makes for different "quarter" seams as well. That's why it's reccomended to buy the same brand for all your rulers.
With your ruler, align your fabric with the 1/4 inch guide and draw your seam guides onto the fabric. There's YOUR specialized quarter-inch seam allowance. If you are lucky, your machine foot will work with that line. Position the needle and foot to work together with that line and sew two pieces together and check your measurements. Sometimes you have to move the needle over just a little to compensate for the fabric fold.
You don't have to draw lines on all your fabric during a project. I do it for test samples on each project and then go from there. Sometimes each project has a different 1/4 inch seam allowance.
Right now with the 9-patch blocks that finish at 1.5 inches, I've drawn lines on each block for more precise accuracy. If the stitching line is off from that line, I redo it. Now that I have it set up right, I don't neccesarily need that drawn line. It's just there to make sure I didn't run off the road.
I hope that helps someone else find their true and accurate quarter inch seam allowance.