ISA FL330 = +15º - (2º x
33) = +15º - 66º = -51ºC deviation = +15ºC, OAT = -36ºC.
In the AIRSPEED window
put the temperature (-36ºC) next to the Mach No Index, go to the TAS (487kts)
on the outer main scale and read the Mach No next to it; 0.81.