how do you deal with "at or above" or "at or below" requirements on charted arrivals and departures?
We have the luxury of a 3rd Altimeter (standby).
On departure it is left on QNH until above TA / MSA (which ever is higher) and is set to QNH before descent on arrival. The AP Altimeter reference is set early to the controlling setting (QNH or STD) (to avoid potential ALT BUST).