Again, thanks!The only reason any setting would be too high is if it is disturbingly audible. Any setting above zero is useful to confirm the fan actually works (although, for that purpose, I would choose 100%).
The setting you choose should be where you find an acceptable compromise between noise and cooling, and you can monitor the effect using the temperature plot in Sysmon. Too low a setting and the HDD temperature will rise until the standard algorithm cuts in and overrides fan. You'll get a cyclic up and down of temperature, which actually isn't very good for electronics and mechanical assemblies.
With sufficient setting (the lower limit will vary with ambient temperature), the temp plot will stabilise at some temperature below the Humax trigger point – depending on the actual setting and the ambient temperature.
FWIW, I have all my HDRs set to 40%, and (with an ambient temperature of 18°C) the HDDs read between 44 and 48°C. If I kicked that up to 50%, the steady temperatures would come down a little.
Note it can take a few minutes for a change of setting to take effect.