Notes that helped me figure out the mapping scheme.

UnprivilegedLXCContainersMappingNotes.txt

To me, it seams like its supposed to remap all of the containers {u,g}ids.
So the important detail is that all of the containers id need to be maped, not the hosts.
Please correct me if I'm wrong.

I added "symbols" to make the syntax more readable, and some show the result on the nest line.

// https://www.reddit.com/r/homelab/comments/6p3xdw/proxmoxlxc_mount_host_folder_in_an_unprivileged/dkncmrc/
0 -> 100000 x 124
0-123 -> 100000-100123
124 -> 1004 x 1
124-124 -> 1004-1004
125 -> 100125 x 64410
125-65534 -> 100125-165534

// https://pve.proxmox.com/wiki/Unprivileged_LXC_containers#Using_local_directory_bind_mount_points
0 -> 100000 x 1005
0-1004 -> 100000-101004
1005 -> 1005 x 1
1005-1005 -> 1005-1005
1006 -> 101006 x 64530
1006-65535 -> 101006-165535

0 -> 100000 x containerid
containerid -> hostid x 1
containerid+1 -> 100000+containerid x 65535-containerid

123 -> 1200
99 -> 1300

0 -> 100000 x 99
99 -> 99 x 1
100 -> 100100 x 123-99 (24)
123 -> 123 x 1
124 -> 100123 x 65411

c1 -> h1
c2 -> h2

0 -> 100000 x c1
c1 -> h1 x 1
c1+1 -> 100000+(c1+1) x c2-(c1+1)
c2 -> h2 x 1
c2+1 -> 100000+(c2+1) x c3-(c2+1)
c3 -> h3 x 1
c3+1 -> 100000+c3 x 65535-c3