I like to think I cut and sew relatively accurately -
and when I'm sewing fabric that is "on grain" - I usually end up with something close to what I'm aiming for.
For the life of me - when I use that plus 7/8 inch and sew with my "usual" seam allowances, those doggone finished squares are ALWAYS skimpy.
So - even though it takes longer, it saves me aggravation and fabric in the long run to cut with the plus one inch and neaten up the units after sewing and pressing.
I haven't done the starching/sizing - maybe that would help?