If you are sewing 6-1/2" squares together to make HSTs, you will NOT have 6-1/2" finished. You need to add the 7/8" to the finished size .... so you want 6" finished, then it'd be 6-7/8 cut sizes for the HSTs .... I always add the full inch to give some wiggle room, and lets me square up nicely.
Likewise the added amounts if you're doing the QSTs