I wish we could come up with a term for where to sew instead of 1/4" seam. You don't measure the seam, you measure the resultant patch!
Here's a tutorial to the method described above:
Achieving an Accurate 1/4" Seam Allowance
Keep moving a needle until the center strip measurement is correct, then write down the settings on your machine and use something, like a piece of tape, to indicate where the edge of the fabric should be.
Two additional comments. Make sure you are pressing carefully and not stretching the seam (can you see the sewing thread?) or pressing a tiny fold at the seam. Also, although you have written the machine settings and possibly marked where the edge of the fabric should go, the fabrics and thread you use can affect this and you may need to adjust! I often will do a quick check at the start of a project to verify I'm still right on. I'd rather spend a couple minutes checking my seam then a whole lot of time ripping or fighting to get stuff to fit.