>>133513and forumula
ffmpeg -i "File location" -ss 00:00:00.000 -to 00:00:00.000 -c:v libvpx -c:a libvorbis -crf 4 -b:v 3072*1024*8/Duration in seconds*0.975 -filter_complex "scale=-1:480,subtitles=file location" -ar 22050 -y Outputname.webm
results in pic related