Ok, I am going out on a limb and making a tshirt quilt for my son. I have sketched it out with 12.5" tshirt squares and then using 3" square in a square blocks as sashing. I just need to know (have verified) how big to cut my half-square pieces to get 3.5" unfinished diamond blocks. I think 2.5" squares sewn together and then cut in half and paired up with another set of two will give me the right size, but need someone to verify it for me. This is what I want the finished piece to look like, but sized to 3.5" square.