I think the 'rule of thirds' goes back a longer way than might seem.
As long ago as 1960, when I was toying with the idea of joining the RN to fly, I was told that the Navy worked on a 33% attrition in aircraft during normal ops/training.
I was reminded recently when reading a book about carrier operations, how there was an elaborate replacement/replenishment system, which involved smaller carriers 'feeding' replacements to the fast fleet carriers. Whilst that was war, there was obviously careful planning for managing large numbers of aircraft to ensure the front line was sustained.
O-D