This was my train of thought. The original image is 720*576, at a 4:3 aspect ratio. You want to go to a 16:9 aspect ratio of 720*x. If the pixel ratio would match the aspect ratio you would expect it to be as easy as this: 720/16*9=405. But they don't match: 720/4*3 is not 576, but 540. So to go from the aspect ratio to the pixel ratio you'd have to multiply it by 576/540. If you apply that to the 405 you got from the previous calculation you get 432. So there's that. But where did you get your 324 from?