These ratios are calculated between the 95th percentile of power levels across the whole spectrum (signal), and the median power level (noise).
If you are a KiwiSDR owner trying to improve your setup, here are a number of leads from WA2ZKD:
The Auto Scale button on the waterfall control tab will also give you instant, real-time signal and noise level metrics - as the WF max and WF min numbers - similar to these scores to evaluate the performance and impact of your improvements as you work on your receiver. "Internal" scores are computed and self-reported by KiwiSDR receivers themselves; you can access those statistics at http://<your_kiwi>:8073/snr You can also use the
microkiwi_waterfall.py utility of the KiwiClient command-line suite to replicate external calculations of such scores by yourself. If you know of, or can contribute, a more comprehensive and in-depth guide to improving and getting the maximum out of your KiwiSDR, contact us and we'll gladly link it from here.
Disclaimer: shortwave reception is a complicated topic, and reducing it to a single number comes with no guarantee of getting a representative or useful result! The scores listed here are most relevant when comparing receivers located in the same area.
Note: Internal (self-reported) scores also vary throughout the day, and are usually higher during nighttime.