6.8% seems like a pretty reasonable target for non-regrettable attrition. GE famously aimed at firing the bottom 10% of performers every year. In Facebook engineering management the internal target was 6% IIRC.

It feels a bit paradoxical because there are many 10-person teams with no poor performers that should be fired. Yet if you have an engineering director heading a 150 person department and that department allegedly has no poor performers, that probably indicates a lax culture that accepts poor performance.