How do I use ffmpeg to decrease the size of videos by 15%?

I have a directory of videos that I need to fit within 29.3GB. The videos take up 34.4GB. How can I use ffmpeg to decrease the size of all videos by 15%? I looked at the options and it provides an option for setting the bit rate, but they are different qualities. I wouldn't want a low quality video to increase bitrate. I tried creating a batch file with

ffmpeg -i file.mp4 small/file.mp4

But it was still too large at 29.6GB.

I'm currently running

$ for x in *.mp4; do ffmpeg -i $x -preset veryslow -c:a copy smaller/$x; done

Ref: https://trac.ffmpeg.org/wiki/Encode/H.264

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