For the A320 family, it’s in the FCOM.
PRO-NOR-SUP-MISC - “Operations at QNH above 1050hPa”.
Essentially you have to check whether you’re exceeding the flight envelope of -1000’ based on STD.
Otherwise, the procedures vary slightly depending on the fit of the aircraft. The old standby altimeters have to be set to 1050 or less with some mental maths added (in 100s of feet). The ISIS fit aircraft can all be set up to 1100hPa (although you’d likely exceed the flight envelope before getting anywhere near that!).
There is a slight amendment to the way the PERF page is set up and possibly a transponder procedure to apply too.
champ