ghostrider

Generally, if you cut "outside the line" of your ruler, you can sew a regular quarter inch and the block/quilt will wind up the right size.

Scant quarter inches are a result of the difference between rotary and template cutting. When we, some of us anyway, used to cut using templates and marking the lines to cut on, we 'left the line', as carpenters say. Meaning the line stayed on the cut piece, so it was actually just a 'scant' bigger than measured.

When those slightly bigger pieces were machine stitched, the width of the line we left was enough to compensate for the thread thickness and the fold of the seam when it was pressed and the blocks came out the size they were supposed to. With the advent of rotary cutting, the cut pieces actually got smaller in comparison because there was no marked pencil line. If you use the inside of the ruler line, your pieces are even smaller.

So, try this. Using the outside of the ruler line, cut two 2" by 4" rectangles and sew the 4" sides together with a regular quarter inch seam. Set the seam (press the stitch line as sewn) and then press it the way you usually do. The width should be 3½". If it's more or less, you'll need to adjust either the cutting or the sewing...or not. As has been said, it's the consistency of your seams that matters in the end.