Large file download progress feedback

[Gary]

I have an audio file that is accessed via a link and I use the
Response.AddHeader method to add the content-disposition header so that
the user is prompted with a "Save As" dialog. I am converting the file
on demand from one audio format to another. This is a long-running
process and I won't know the resultant stream size in advance. This
means that there is no feedback for the user until the conversion is
finished. Is there any way I can provide some feedback as soon as the
link is clicked? Should I just set the stream size to a guess?

Cheers

Gary