I think I've cracked it.
The assumptions so far in all the posts I've seen are that the numbers represent the length of the *entire* track. I've just been toying with the numbers from a "Point at which this track can be interupted" angle, and it seems to be working. Keeping the measure count high on the ambients and low on the battle scenes (so that they clear quickly once the enemy are gone) with corresponding values would appear to work. It is not exact, but it's pretty darn close
I'm still fiddling, but :
(Length of track in seconds x Time in seconds between beats) x ((Encoded Bitrate x 1024) x (Length in Seconds / Time in seconds between Beats)) / 2