With years of manually controlling cabin pressurisation in old turboprops, the following might help:
The A320 family manual control of the pressurisation is time consuming so KEEP IT SIMPLE.
Imagine that the outflow toggle switch is connected directly to the cabin VS. ie toggle up, cabin VSI needle will move up (not necessarily climb) - toggle down, the needle will move downwards (not necessarily to a descent)
Yes, for cruise at or below FL200, you can select a cabin alt of sea level, but why would you?
In order to reduce your workload on the approach, why not select a cabin alt of your platform altitude for the expected approach (unless of course you are landing at a high up airport).
if you maintain a cabin alt of, for example, 3000', once the ac is at 3000', you can open the Outflow Valve fully to depressurise the ac and then the cabin will descend with the ac at a comfortable 700 fpm; this will leave the PM free to carry out his primary monitoring duties.
Once on the ground, you should see 0 diff but you could also crack a FD window to make absolutely sure of that.
i hope this helps.
mcdhu