My apologies for not being clearer. My scenario was confusing.
I'm saying that for every *minute* of DVR space used/elapsed, "x" amount of a movie (in minutes and seconds) elapses. That is, the faster the minutes of the DVR are ticking down (i.e. more DVR space used), the higher the average bitrate of the movie. Of course, to get that bitrate as an actual number (i.e. kbit/sec), the data transfer speed of the DVR/hard drive would be needed. I believe this is possible when using an external HD/DVR if the data transfer speed is known..
That is, in the case of the past few nights, approx. 3 minutes and 40 seconds of movie elapsing were occurring for each minute of DVR used. Today, only 2 minutes and 45 seconds. Thus, DVR ticking down faster/higher average bitrate today.