A320 manual pressurization control

[bringbackthe80s]

Hi all

when dual cabin pressure sys fault the table you get on the ECAM gives you very approximate gates for the cabin altitude given aircraft FL. You should aim at around 300 ft/min in descent, but what other rule of thumb can be used? a specific Delta P maybe? when would you fully open the outflow valve on short approach, literally just before touch down?

Thanks