one-size-fits-all limits are absurd
Engines are different.
Specifying ranges of contamination which don't discriminate between different engine technologies is obviously silly. Clearly a piston engine with a filtration system differs from a jet without. One jet design is likely to be relatively immune by comparison with another.
The limits need to be based on the engine, not the airspace. Data from the manufacturer please. Not recommendations, firm and clear limits.