Ohhhhh... the formulas are VERY easy...
Total wanted Size = e.g. 800 MB
form this you substract the size of the audio (=audiobitrate in kbit/s multiplied by the length of the movie in seconds multiplied by the nr of streams)
now you got the size, which the video should have to fill, added to the audio stream size, the given total wanted size (e.g. 800 MB)
If this video size is for example 570 MB:
570 MB * 1024 = 583680 kilobyte
583680 kb *8 = 4669440 kilobit
But as the Bitrate is measured in Kilobit per Seconds:
Divide the video size in Kilobit (in my example: 4669440 kilobit) by the length in seconds (e.g. 7200 seconds = 2 hours):
4669440 kilobit / 7200 s = 648,53 kbit/s
Now there you've got your Bitrate
ATM I am trying to teach myself C# (C Sharp)... maybe I'll code another bitratecalc in a Programming language whichs projects don't take that much space