Minimum recommended free space?

[eton]

What's the minimum recommended free space on RAIDiator 4+ that does not compromise performance too much?

[eton]

Something here, but no real answers:
viewtopic.php?f=7&t=19601

[vandermerwe]

I've never seen any structured assessment of performance vs. volume "fullness", however it's generally suggested that above 80% is considered full as you are likely to see more fragmentation which will affect performance.