10" finished = a 9 patch needs to have 3 1/3" finished squares - not an easy thing to play with. Will the finished project allow for a very narrow sashing? If so you could make the 9" finished block with a 1/2" tiny frame (ending up a 1" between blocks) and end up with your 10" finish. Only other suggestion would be to just go with the 9" finish and plan on extra 'real' borders to make up the difference.