It looks like you're doing an excellent job. I would continue with the measurements you've been using on this quilt and worry about your 'scant' 1/4 later. As long as things are lining up with your current settings, there's no need to worry. I am a self-taught quilter. Mostly (like many of us here) from watching Simply Quilts with Alex Anderson on HGTV. She had on a guest at one point that stated something to the effect of using 'your PPM'...your 'personal piecing measurement'. This guest, if I remember correctly, was a leading log cabin quilter. Her feeling was that as long as your seam measurement was consistent throughout the project, getting that elusive 'scant 1/4" ' was not nearly as important as all of your seams being consistent. Words I have lived by in my quilting career!