If you sewed 4 strips that were 2.5", you strips set should be 8.5" or thereabouts. I say thereabouts because depending your your 1/4" foot and your experience you could be off by a hair or 2 either way. As Bellaquilts asked, what do you mean by off? Are you over or under 8.5", and by how much? If you're within 1/8" (8 3/8" or 8 5/8"), that's not totally unheard off and is usable. Give us more details if you can.