Recommendation on RDS Postgres "Free Storage Space"?

How much free storage you should maintain mostly depends on your use case.

RDS storage autoscaling feature, when enabled, triggers when free available space is less than or equal to 10 percent of the allocated storage: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIOPS.StorageTypes.html#USER_PIOPS.Autoscaling.

To control cost, you may want to proactively monitor RDS storage as explained here: https://repost.aws/knowledge-center/diskfull-error-rds-postgresql