Thanks for the answers about the cold weather corrections.
It looks like you are correct in Airbus wanting the pilots to do it all themselves with this statement from the FCTM....
"FPA Correction
When the temperature is lower than ISA, the FPA that the aircraft actually flies is less steep
than the FPA that the ADIRS (ISA referenced) computes.
In vertical selected mode FPA, to correct the FPA for this ISA deviation effect, the flight crew
should select on the FCU a FPA slightly different from the FPA that the aircraft needs to fly."
Bottom line, you have to guess at the descent angle correction.