Is there a rule or something I can use to calculate a good number for
The defaults are odd. PostgreSQL defaults to max_connections=100 while pgboucner defaults to default_pool_size=20. Shouldn't default_pool_size always be higher than max_connections? Otherwise, what's the point? I thought pgbouncer was meant to let us handle more connections by lowering their overhead (by reusing PostgreSQL's connections). I'm confused.
I'm looking for advice similar to the ones found in PostgreSQL's wiki, like "this parameter should be ~50% of your memory".
I also remember there was a spreadsheet for MySQL that would let you calculate these kind of parameters. It would be awesome to have something like that for PostgreSQL/pgbouncer.