The only benchmark I've seen is for internal or external communities, created by the Community Roundtable, their latest is available in their State of Community Management 2017:
We only have one internal "learning community" and colleagues have to participate as part of their learning assignments.
The instructor will propose a question, and the participants HAVE to reply and argument their response.
So it's mandatory participation, not really a community where colleagues exchange, ask questions or learn from one another.
From your stats: 25% +50% + 20%, it's almost already 100%... of engaged members. That's a lot I think.
I would imagine that at least 25-42% would be either lurkers / listeners or inactive according to the Community Roundtable benchmarks of 2016 and 2017 (p.10)