It also looks nice for 20% (4h HBD per v HIVE), but for 30% we get 2h HBD per v HIVE - exactly the same as for 33%.

Is there anything wrong with using 20% or 25% or 33 1/3% and keeping the "couple of lines" of code change minimal? The number seems mostly arbitrarily picked, I don't see the value in fretting over getting right to 30% vs 25% or 33 1/3%?