Because real life has no relation to the ideal, I add 1/8 to the measurements and then trim to size. So, for a 3X6 flying geese, I make one 7 and 3/8 inch square, and four 4 inch squares.
Believe me, I trim less than 1/8 inch!
add 1 inch to the short measurement for the 4 blocks, which will be the 2 small triangles, and 1 and 3/8 inch to the long measurement for 1 block, which will be the large triangle.