>>29312101 (OP)
>I am trying to find the best possible combination for FFMPEG to convert a webm video under filesize and still have the best possible bitrates + resolution
https://www.videohelp.com/calc.htm
Manually calculate the bitrate
The basic bitrate formula is
(Size - (Audio x Length )) / Length = Video bitrate
L = Lenght of the whole movie in seconds
S = Size you like to use in KB (note 3,95 MB x 1024Β° = 4045 KB)
A = Audio bitrate in KB/s (note 48 (opus) kbit/s = 48 / 8Β° = 6 KB/s) - or less Bitrate
V = Video bitrate in KB/s, to get kbit/s multiply with 8Β°.
Β°8 bit = 1 byte.
Β°1024 = 1 kilo in the computer world.
Example
10 Seconds video,
4 MB File but be sure that if fits use a bit lower like 3,95 MB, S = 3,95 x 1024 = 4044,8 ~ 4045
Audio bitrate, A = 48 kbit/s / 8 = 6 KB/s
(4045 - (10 x 6) ) / 10 = 398,5 KB/s x 8Β° = 3188 kbit/s.
The best way to ensure that the exact file size is maintained is to use 2pass.