22mm i believe is the minimum thickness... at least that is what the FSM says for my 2002 Forester, with similar specs for a 16" wheeled 2006 Outback... the 17" wheeled version has slightly larger specs, so I would suggest checking the FSM for your vehicle
per the 2002 Forester FSM...
Standard value - 24mm (0.945in)
Service limit - 22mm (0.866in)