[n0mag!c]

Guys, I hope Atak_Snajpera will not be so lazy to share his formula with us.
Till then I'll be not so lazy to share my knowledge with you. Many years I use this formula for raw stream of any type:
Smb=Tmin*Bkb*0.0075, where
Smb - size in megabytes
Tmin - length of stream in minutes
Bkb - bitrate in kbits/s
0.0075 - is enough precise factor, you can easily try this formula for yourselfs.

We must use different CONTAINER FACTOR for different containers. And muxing test gives me 1.065 result for m2ts container.
for example: Total bitrate (kbit/s) will be 4700(media size) / CONTAINER FACTOR / Tmin / 0.0075.
Then video bitrate will be "Total bitrate" - "audio1 bitrate" - "audio2 bitrate" - ...

So G_M_C is quite right when using 7-8%%, especially if we wanna use subtitles...