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.

When you make 2 HSTs by putting 2 squares on top of each other and sewing 1/4" from the center, the general guideline says to make the squares 7/8" larger than the finished size of your HST. Your finished size is 6" so you would start with a square at 6 7/8" - HOWEVER, I find that the 7/8" does not give any leeway and I have to be super precise in all aspects. I much rather cut my squares 1" larger than the finished HST and square them up. Yes, it is a bit more work but worth it to me.