Use the texture magic with the batting and fabric the same size. ( not sure if I am explaining this properly) The finished "piece" ( fabric , batting, and fabric) or unit will be approx 25-30 percent smaller. The demonstration I saw said to be safe use more like 22 percent smaller. It will shrink in all directions. So if you need a 10 square you need 13 inches of all , fabric, batting and texture magic. this is figuring ruffly 30 percent.
Different fabrics will react differently , as will the batting ( if used ) will affect the final diminsions.
Take the lenght of the desired finished piece and mulitlply by 1.30 then take the width and do the same. These are your new starting dimensions.
I have been using 25 percent with most of my fabrics and so far so good . I have a feeling I may get a bad surprise some day. Its so expensive I just do not want to waste and inch.