Users can see how the witnesses answered this question on the PeakD witness page.
https://peakd.com/me/witnesses
It appears that the witnesses can simply type in a number.
The number of witnesses that vote for exactly 20% is interesting. This implies that there are whales who want 20%. My guess is that witnesses have to vote 20% to get whale votes.
As for the answer to the question: To maintain a balance, the interest on HBD should be significantly lower than that paid on HIVE.
Since interest is not inherently important to HBD, I would vote 0%. But I suspect that the public at large would prefer something like 10%.
!WINEX
Vote what you think is the best (you can do multiple votes using dpoll), well maybe it's 20% atm but little after little let's change it