OK, so you have 704x342 (after cropping all 4 black borders).

Considering PAL PAR, that gives us 768x342 square pixels, so the DAR is ~2.24. We want to maintain this DAR so that the anamorphically-encoded DVD video, when stretched on your widescreen TV, will have the same proportions as the source video.

Now, notice that 854 width that I was throwing around before. That's for NTSC. You get that by doing the following calculation: (480 * 16)/9 = ?.

For PAL, we do a similar calculation: (576 * 16)/9 = 1024. So we now know that when the video is displayed on your widescreen TV, it will have an approximate resolution of 1024x576.

Using the same method as before, we need to adjust the height. The calculation is (342 / 768) * 1024 = 456. So, you need to make the size 720x456 (before adding black borders) in order for it to look correct on your widescreen TV after the anamorphically-encoded DVD video is stretched.

(Notice that 1024/456 = ~2.24! This is our DAR!)

How much do you need to crop from the top to get 342?

I can write the correct code for you if you get me that value.