VBR support in MAD

Is there any support for VBR headers in MAD?

I'm trying to calculate the length of files and for CBR
fileSize/(header.bitrate/8) works fine. For VBR, this obviously gives an
incorrect answer.

If reading VBR headers is fiddly, I was also thinking of checking the bitrate in
say 10 or so frames at the start, take their average bit rate and use that in
the calculation above. Would that be even remotely accurate for VBR?

Thanks,
Erik