Is it possible to give nonexact value as Content-Length?

I am streaming a file. I don't know the exact size of it before I end streaming, but I know an approximated value. The exact value is never bigger than 1 kB then my approximated value. I tried using my approximated value for Content-Length but Firefox ended downloading when it reached this value so it did not get the last ~1kB even thought it was still streaming.

So I want to let the browser know how much data to expect. Can I do that?