I found that page about 2 months ago. However, see the caveat at the bottom:

For larger files it's using something different - not sure what yet.

This is what's causing us the difficulty. I've got a program that works out where the frame headers are and then correlates them to those stored in the header. The error numbers are suspiciously round, but the deltas involved are by no means regular.

We're hoping that someone will just hand us the spec (even under NDA would be fine), and then we can deal with this properly.