I agree with over cutting -- adding 1" instead of 7/8" to make the HST. Was watching Fons and Porter the other day and they mentioned they do the 1" since if you are using fabrics that are heavier than the one the pattern was designed with your squares may be a bit small after the HSTs are ironed.
That said -- if I need 7/8ths, say 6 7/8, I start with the ruler in front of me and starting at the 7, I then move one hash mark to the left (toward the 6) and that is 6 7/8.