I can't find anywhere on the offial support area about the 'Peer read caching' option in Settings-Transfers-Local Files.
I'm trying to get the best setting on a high end computer, any help?

I think this refers to torrents are seeding.
So what would be the best setting to save disk usage?

While there is a page for "Local Files" https://support.tixati.com/settings%20-%20transfers%20-%20local%20files   there is no longer any such Settings option with that designation.

The "Peer read caching" option is now found under a renamed and reorganized Settings/Transfers/Files  
A page for that is underdevelopment at https://support.tixati.com/settings%20-%20transfers%20-%20files

Neither page explains this function and as you point out, no authoritative discussion of this option can be found as yet on either the support pages or the forum.

on high end computer you dont really care, because OS will put seeded files into ram cache anyway, not to mention these settings are useless if you cant tweak cache size and other related disk io settings, use whatever you want, you wont notice difference :)

I have couple of 5TB to 8TB USB3 hard drives full of torrents that I seed.

Configuring this setting really makes a difference. However the best setting changes
if people are downloading more of the recently downloaded or not yet completed fragmented
files, or if they are downloading older not fragmented files.

I really would like to know what "predictive" and "strong predictive" settings are really doing.

Also I would suggest to add better configuration here, because read caching for torrent that has
64kb pieces vs torrent that has 8mb pieces. Maybe predictive is exactly trying to figure out the
best policy, but it seems to raise HDD usage % to very high numbers. Perhaps exposing parameters
of predictive caching would allow to find better configure.

Any way, this setting really is important if you seed huge number of torrents, even more if you do
it by slow USB drives etc.

And supporting seeders should be the main priority for obvious reasons.

Keep caring and sharing =)