Abnormal landing distance 320/321

[Speedwinner]

Hi folks,

Quick one on that: as we often fly really heavy 321 out of far away destinations, do you have some thumb values at which weight we can return to the field of 3000m. I know it depends on the failure but the most critical is double hydraulic failure and the 321 gives there 4500m required. So when to start the approach? Burn it down to max landing weight? In a mayday case? Any values for 320 or 319?

Thanks folks