theora - Jan 2007 - Bitrate expectations for encoding or decoding

Hey Folks.

I'm currently working on implementing Theora Video playback for the  
Unity engine (http://www.unity3d.com). So far everything works fine,  
But I have a small question:

Is there a reliable way to predict what bitrate a stream will have  
when encoding or decoding?

Of course, there's the target_bitrate parameter in the Theora Info  
structure, but somehow, I always get quite different bitrate results  
for my encodings (typically about half as much). Is the quality  
parameter ignored when target_bitrate is non-zero? I'd like to have  
some idea of what I will get, so I can present the user with a file  
size estimate before starting the encoding. What would your  
suggestion be on how to do this.

Also, we support streaming Ogg files from web sources. Now, I'd like  
to be able to know if we have enough data, so it would be ok to start  
the movie given the current download percentage and download rates.  
But the problem is I have no idea how to predict the actual playing  
time of a movie. Unless I overlooked something, there doesn't seem to  
be any relevant information in the Ogg headers (as it's a streaming  
format). As I know the file size, i could estimate the total playing  
duration if I knew the bitrate, but as stated above, the  
target_bitrate field is not really meaningful (and it may be zero, if  
quality is used).

So, my question is: is there any reliable way to predict what bitrate  
a theora stream will have before encoding or decoding?

TIA,
jonas