I always add 3/8 to the finished size. If I need a 3.5" finished square. I cut my square to be halved 3 1/2 + 3/8 = 3 7/8, cut in half. Sew haves together and you get your 3.5" square.
So if the 1" is your finished size I am assuming 1.5 would be including your seam allowance, so your squares to be cut in half need to be 1 7/8.