For the 12 blocks you will need 48 strips 2 1/2 by 6 1/2 of each color. If you cut the strips at 6 1/2 in you should be able to get about 18 2 1/2 by 6 1/2 strips. You would need to cut 3 strips at 6 1/2. The amount needed would be 20 inches. Does that sound right?