How to calculate/estimate the container overhead of flv?

I want to be able to calculate a target bit rate for a two pass encoding to hit a specific file size.
I can do this for avi/mp4/mkv/mov/m2ts (at least least relatively accurate) and there are quite a few sources out there talking about the container overhead for these containers, but I no discussion about flv container overhead.

As video streams I would use: MPEG-4 AVC, VP8, VP9
As audio streams I would use: mp3, aac, wav, speex
Muxing would be done with: ffmpeg

I know of the Adobe Flash Video File Format Specification and I know that knowing this specification and the ffmpeg source code it should be possible to come up with a 'rough' container overhead estimation, since this is a lot of work and I probably rather pass on flv as a container than trying to figure this out.

Since I couldn't find any info using Google, I'm interested in this issue for quite some time and it doesn't seem to be too far fetched that someone already figured this out:


Or are there at least some folks that can share some 'rule of thumb' ways they use to estimate the overhead?

Cu Selur