call up the Plex XML info of the file in question
take a look at the URL bar. It contains a parameter &includeExtras=1
change this to &includeBandwidths=1 and press Enter
You will get the detailed analysis results for the overall file and also for each substream.
requiredBandwidths="27166,26312,24247,22347,21003,19838,15765,14563"
The first number is the required bandwidth in the worst case, if your Plex client has only a tiny network buffer.
The various numbers are the required bitrate for 5,10,25,50,75,100,250,500MB network buffer sizes.
You can export this info with ExportTools. Just make sure to use the latest version of it and a detail level of at least 5 (for movies).
If your source file is in the HEVC codec, plex will add a safety supplement to its bandwidth, because it transcodes the file to H.264(AVC) which is less efficient than HEVC and so needs more bitrate to maintain a certain visual quality.
