Your problem seems to stem from a refusal to accept that when manufacture's say there is no safe level of ash ingestion, they mean it. The more ash, the higher the probability of damage, there is no 'safe' cut off level that you seek.
Peter
I am just a practical pilot who has flown all over the world in everything from sandstorms to severe weather.
Firstly there is nothing in aviation which guarantees 100 % safety. If you want that then dont fly FULL STOP!
To date in over 50 years of operation in ash of differing density levels there has never been a fatality caused by inadvertant entry into ash polluted air.
The same cannot be said of other threats to aviation.
Do we ground aircraft because surface winds or shear are at a level that 100 % guarantees that NO aircraft will crash on landing or takeoff? NO
Do we fly in thunderstorm areas where we are 100% guarnteed we will never ingest hailstones, severe turbulence or lightning strikes?
Do we ground aircraft because of the migration season or in areas where large sea birds fly! No think of the HUDSON accident.
I could go on and on with other examples we DO accept and DO have fatality records over the years.
I am as much into safety as anyone but I am also a realist and a pilot and there has to be a sensible approach to dealing with ash which is not apparent by some of the panic statements made by some here.
In my book in threat areas dont fly at night, use basic piloting skills of see and avoid (dont fly into visible cloud or mist patches especially pollution looking ones) and use sensible ash density levels which until proved otherwise allows some movement around known ash areas.
As stated if you want 100% stay at home, lock the doors and probably die of inactivity and boredom.
Pace