The quiltville site doesn't explain where the 1.41 factor comes from. If you have a 1" square, and cut it into a triangle, the length of the diagonal is calculated from the square of the sides, so 1 squared + 1 squared = 2. 2 is the squared length of the diagonal side (hypotenuse), so you have to take the square root to get the length, and the square root of 2 is 1.41 .
Changing the length of the sides, the 1.41 becomes the factor to multiply by the length of the sides.