Ii would cut all my squares to the next whole size and trim the sewn HST units instead of fighting with the 7/8 measurement.
Example: If you wanted a "finished" 8 inch HST units - cut the squares 9 inches - sew - cut - press - and trim.
I consider myself a fairly accurate sewer - but when I use the X-7/8 inch measurement to cut a square - those HST units almost always end up skimpy. I'd much rather trim off a smidgen than be skimpy!