I did some tests, in particular, I converted a video with Final Cut Pro 7 using the value "100" to get the correct aspect ratio and then I imported this video on FCPX and I have another video cropped superimposed on top of the "correct".
I can confirm that the correct size value on FCPX for anamorphic video files 2X of "Y" are 25 / 25 / -25 / -25.
Andrew, thank you for the advice but unfortunately the values are very high and 270 pixels etc do not match.
[b]
HERE THE FINAL CUT PRO 7 FILE "100" aspect ratio[/b]
[img]http://dl.dropbox.com/u/4129710/-100fcp7OK.jpg[/img]
[b]
HERE THE FINAL CUT PRO X (Y 25 / 25 / -25 / -25)[/b]
[img]http://dl.dropbox.com/u/4129710/FCPx-25.jpg[/img]