Anamorphic distortion in FCP

Hi there,

I've got an anamorphic quicktime file that I'd like to generate a 4x3 letter-boxed quicktime file from for web usage. Normally I'd just drag this file to a 4x3 timeline in final cut pro and it would resize perfectly, however I've done this and although the file is letter-boxed there is some slight vertical stretching going on i.e. long faces.

The distortion box is set to -33.3% at the moment - I was just wondering if there was another field value that would correct this... At the moment I'm trying it by eye and I've settled on -42%, but obviously I want to make sure it's right for the client (and not just say I did it by eye if they ask!!!)

Any help would be greatly appreciated!
Thanks,
James