I have a CRS312-4C+8XG. I want to make it quiet. So I started experimenting with the Minimum Fan Speed Percent setting. It looks like the lowest setting the system will accept, and still run the fans is 20%. It does accept 0%, which makes the device silent.

Fan Min Speed Percent Fan RPM
50% 7725 -> 8205
40% 6300 -> 6705
30% 4545 -> 4860
20% 3975 -> 4260
10% 3960 -> 4230
5% 3960 -> 4230
0% 0

Question - How accurate are the RPM readings? If you trust them, can you please enable 10%, 5%, 1% settings? I'd love to have the fans constantly turning at about 400RPM. I feel that would keep my system cool, and be silent.

CRS312 uses DC voltage driven fans. There is a minimum voltage below which the fan is unable to start and would trigger stall protection:
https://cqbluejay.com/what-is-motor-stall-protection/
You could avoid this with a script, that kick-starts the fan with higher voltage and then throttles them down to sustain a lower voltage / speed than required for start-up.

I'd love to have the fans constantly turning at about 400RPM. I feel that would keep my system cool, and be silent.

You need a much bigger fan to meet that expectation. 120mm kind of thing, through a matching hole punched directly into the case lid.