Originally Posted by Chris Scott
Regret to say that I cannot calculate the flight-level where this would occur
For flights longer than 2.5 hours the cabin altitude is 7350 ft (*), pressure 11.19 psi. The safety relief valve setting is 8.85 psi (*), so it would open when the ambient pressure passes through 11.19 - 8.85 = 2.34 psi, i.e. FL429.
EDIT:: (*) Reference FCOM 3.01.21 p.1; Operating Limitations; AIR COND/PRESS/VENT; Cabin Pressure.