For me I use 5 breaths then dump, suck some dill in then repeat until at 6m. Then switch cmf valve back, dump and inject pure o2 until surface. This I have drawn from my own experimenting on the surface and simulating having to go SCR for a ppo2 monitor failure.
Even during a fairly hard swim my ppo2 never dropped below 0.17, and since in a real situation I would be on o2 at 6m its not really an issue (unless I forget of course
)
I had a battery failure once and had to use the procedure for real, it was a bit scary but you've just gotta trust the physics or bail OC.
If your on mccr why do you not have facility to inject 100% at 6m ?