I measured with a very precise Incra rule ( http://www.incrementaltools.com/category_s/157.htm ). With size 50 thread and cotton quilting fabric, the thread takes up about 1/32-inch. So I get a perfect piece if I sew at 7/32, or just past the right edge of a pencil marked seam line. I fiddled with my needle and foot until I had the settings right and then marked the front of the foot. This is my New Home (Janome) F foot.

My ancient LittleFoot was letting the fabric slip all over the place.

That IS annoying. I bet the authors are so used to just skimping on the seam they forget that it's not really a quarter inch and that newbies could not possibly predict this. They all seem to copy the same "YMMV" disclaimer.

YMMV- does this mean "your measurement may vary"?

Do you find that the guide moves a tad while sewing two layers together. It seems that mine moves outward a tad on occasion. I bought a 2nd foot because I thought mine might be defective. But it also seems to move a tad outward. I can't live without mine and just deal with it as it really doesn't effect the outcome as the fabric isn't going under the guide and making the seam much larger, just a tad outward and you can see it and if you measure it is off a fraction.

We see all the time how many woman sell items they thought up and they make good money. Now, one of us needs to come up with a scant 1/4" foot for many machines (Bernina please) and I promise that you will make a lot of money. Either have your husband do it or find someone who can do it and do a trial run to see how many (thousands) people will buy one or more. I guarantee you'd be able to retire!! I only wish my husband was that handy. But there are places you can go to with your ideas and they will bring them to life. Look it up online.