If I understand correctly, you want a block that is divisible by the size of the finished block in the round. So... If the block in the round is 3 1/2 inches unfinished, it will be 3 inches finished (due to seam allowances). Then, your center should be 36 1/2 inches (the extra 1/2 inch is for the seam allowance). If your block is currently 33 inches, you need to bring it up to 36 1/2 inches. So... Adding 3 1/2 inches means 1 3/4 inches added to each side plus 1/2 inch total for seam allowance. 1 3/4 + 1/2 = 2 1/4 inches added to each side. Please see if others agree with me.