Using actual atmospheric Pressure Lapse Rates (which are logarithmic and not linear), in this case the correction is very close to 27.5 ft / hPa.
That makes the Pressure Altitude at F/L 120 equal to 11581 feet, giving you 4681 feet clearance above your 6900 ft (2103 M) mountain IF the atmosphere is ISA temperature.
At lower than ISA temperatures, the clearance will be less, the opposite for above ISA temperatures.
Regards,
Old Smokey