Distort- Aspect Ratio Math

Hi: I am trying to figure out the math for the various Distort/Scale Values entered when resizing footage of various formats to put into various timelines. I understand where the Scale value comes from (say putting DVCPRO HD 720p60 (960x720) into an NTSC timeline (720 x 480) Scale = 75 = 720x960, expressed as a percentage is 75%. But the distort needed, -50, can anyone explain where that comes from and how that number can be explained mathematically?

I have done a lot of combing through various posts but can't seem to find that information. If I am missing it, can you point me towards the thread?

Thanks,
Lara Johnston