Sort:  

Yea so it's the median price between top20 + 1 backup.
There have been discussion since I've convey your question to other witnesses and there could be an update to do a median price between top19 witnesses instead in order to get a more stable APR.

I appreciate you looking deeper into this and discussing it!

I'm wondering whether a mode might be a better approach? It seems like it would be more reflective of the general consensus, unless every witness decides to switch to a different number - which seems rather unlikely.

I did suggest a mode but another witness came up with a counter example where it would not work as expected. Say 18 witnesses each have different APR values and two of them have an APR of 0%, mode would result in 0% APR.

10 witnesses signal 20%, 10 signal less. The APR is calculated each round anew, so currently it depends on the chosen backup witness if it's 19% or 20%

I’m not 100% sure of how the calculation is done but it could be a median over last several days or median from witnesses APR value.