Sometime we have to see things to know the reason. I suggest you take two 6.5"squares and sew diagonal, cut and measure. Do the same for 6 7/8", and the 7". You will quickly see and understand the difference. Even if you do a 6. 7/8" and sew diagona with 1/4"seam, cut press and measure you will know what adjustments are necessary for accuracy.