innodb_buffer_pool_size on a machine with other tenants

The advice to set the buffer pool to 70-80% of RAM assumes a dedicated database server, and this one also runs PHP and Redis.

the host: 16 GB

  php-fpm, 20 children × ~90 MB    1.8 GB
  redis, maxmemory                 1.0 GB
  nginx, os, everything else       1.0 GB
  page cache, needed for the OS    1.5 GB
  ────────────────────────────────────────
  available for innodb            ~10 GB
  set to                            8 GB

and the number that says whether it is enough:
  buffer pool hit rate, 99.4%

The hit rate is what should drive the number, not a percentage of total memory — 8 GB at 99.4% is doing its job, and adding another two would buy a fraction of a per cent while pushing the machine toward swapping. Working it out from what else runs on the box takes ten minutes and is more defensible than a rule of thumb from a single-purpose server.