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I personally have never been able to sew a perfect block, with perfect seam allowances, which effects how the whole finished quilt top sews together, as each 1/16th of an inch can add up to a significant amount in the finished size.
What I came up with early in my quilting, was to draw a line at the 'finished size', with a mechanical pencil, on the back of each unit or block. I use this sewing line as a guide to pin to the next block, plus to sew along. The final result may have the blocks slightly off, or 'charming' in the quilt top, but the final quilt top itself lays flatter and in the end the finished quilt is nicer.
My 'Blocking' how-to file, that describes the technique step by step, is available in the Techniques ~ General Directions section, in the Library of my site here.
I lay the unit or block with the wrong side up onto the sandpaper (or rough section of my cutting mat if the block is larger), and center a ruler (the finished block size I need if I have one, otherwise I use a ruler the closest to the size) onto the back of the unit.
Draw a 'sewing' line, with the mechanical pencil, around the ruler.
This gives you good corners and makes up for any uneven sewing.
You can trim any extra seam allowance, if you wish to take the time.
Pin your blocks together using this Perpendicular Pinning trick:
You then stitch them together, sewing edge to edge, along the line you have drawn.
When you combine both of these tips I promise your quilt tops will end up neater.