Are you allowing for your seam allowance that will come off the 7 inch block? On your side pieces you will have to have a seam allowance on both sides of your side pieces also. I believe that you need a 4.5 piece, 7.5 t-square, and another 4.5 piece to equal your width of finished 15 inch block. If you cut your t-shirt blocks 7 inches it will finish at 6.5, so in that case your Pieces need to be 4.75 piece, 7 in block center, and another 4.75. I could be wrong, but this is the way I would figure it. Hope this helps.