In your 720×480 Anamorphic 29.97 timeline, simply place your 640×360 clip and scale it to fit the frame. Don’t use distort. FCP will scale the frame and pixels to make it anamorphic 720×480. If you absolutely must do it outside and use a software that doesn’t know about non-square pixels, then you’re right. it will have to be 853×480. Your math is right.
When I’m authoring NTSC DVDs and menus need to be 16:9, I often create graphics as 853×480. There’s no other way.
In PAL it would have to be 1024×576. That’s the ratio we use when exporting from a software like Resolve and we need it to go into a PAL anamorphic project.
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Neil Sadwelkar
neilsadwelkar.blogspot.com
twitter: fcpguru
FCP Editor, Edit systems consultant
Mumbai India