10" squares cut in half twice will yield 9" FINISHED four patches (9.5" UNFINISHED)
10" squares cut on both diagonals will yield 8 3/4" FINISHED QST (quarter square triangles) (9.25" UNFINSHED).
And that's if all the sewing is perfect. And if you're off the 1/4", then it was. ;-) Which, strangely, doesn't help you. :roll:
(There are no HST blocks being made.)
You can trim the 4-patches down to 9.25" UNFINISHED ,or trim them all down to 9" UNFINISHED. ALL the block would FINISH at 8.5".
How annoying that the pattern is written that way.