I'm going to be very blunt about this. It was 72% at the federal referendum. That could have been considered satisfactory. Then it was 72% in Great Britain, and that was unsatisfactory.
The point is, how important is the question to society? That has an impact on the turnout and the legitimacy of the results. It cannot be a simple answer of just one number being reached. The gravity of the question weighs on what an acceptable participation rate would be.