Thanks for the excellent, simple recommendation.
For everyone’s edification, I am editing in SD 4:3. I wanted to create a 4:3 mask around the actress’s mouth, and have it follow her mouth using motion tracking. After I have a 4:3 mouth video for her dialog, I intend to use Red Giant to expand the mouth to 720×480 SD so that it fills the screen.
(We were rushed out of the location, so I never got to take an extreme close up of her mouth. This is my workaround.)
So I enabled the grid, set it to 10 pixel squares, and dragged to create a square mask. I aligned the square to the grid, which made the sides about 98.9% per side in the Scale parameter. Expanding the Y coordinate by 33.3%, to 132.2% in the Scale parameter, gave me a 4:3 aspect ratio.
mac osx 10.6.8
Final Cut Studio 6