It's more than just the PPO2, it's also the total exposure time as well as other predisposing factors (Co2 retention, high levels of exertion, caffenine, etc.).
As a general number I think 1.4 is "reasonable", but that has the same meaning as a dive table being "reasonable" for safe decompression. People still get bent now and then even within the table limits, especially if they push the limits, use the table beyond what it was designed to do (week long multi-day repetitive dive profiles, etc.) and add some predisposing factors such as dehydration, etc.
In short there are a lot of unknowns and it makes sense to recognize what may increase your suceptibility, be aware of symptoms of an imminent O2 hit and adjust your conservatism accordingly.