I know what you mean about that scant qtr inch! When I was new to quilting I would measure, remeasure, cut perfect and sew a perfect qtr inch and my blocks would always be off by an 1/8 -1/4 inch, grrrrr. No big deal if I was making blox for myself, but when swapping with guild members it is another thing....many want perfect blocks!
A friend told me that instead of trying to do a scant 1/4 inch by eye on the machine, she said if my machine would do it, to move my needle to the right so that it would be a 'scant'... and you know it worked! For the first time, no frustration, my blocks came out perfect, sometimes even a smidge big and easy to square up.
Machines are different and some won't allow you to move your needle. I have a Janome that has my needle set at default of 3.5 and I click my button so that my needle moves right and my screen shows that I have it set at 4.5. (I have a qtr inch foot that has a small groove on both insides, when my needle is at 4.5 it is near the edge of the groove without my needle hitting my foot and breaking the needle) If your machine won't let you move your needle, take some blue painters tape and using a exacto blade, cut down about 4 or five layers and cut again maybe 2 inches away so that you get a 2 inch or longer strip of tape layers and tape it on your machine by your sewing area but less than a 1/4 inch. That way, your fabric will butt up to the tape easily and you don't have to worry too much that your seam will vary. Voila perfect scant 1/4 inch seams..
hope this helps!
sue