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tl;dr: don't compute the pool size, set it to a big number, the pool size is the max that Rails enforces, but the thing that matters is the number of connections available at the database (which is a separate issue).

If, rather, you're running out of connections at the database, then try things like:
- reduce the number of Puma threads
- reduce background job threads (e.g. via GoodJob, Solid Queue, etc.)
- "Configure anything else using a background thread making database queries"
- among others

Or increase the number of connections available at the database with a tool like PgBouncer.

This post was written by the person who created GoodJob.