Final answer:
Above 18,000 feet MSL, the altimeter should be set to the standard altimeter setting of 29.92 inHg or 1013.2 hPa, which is used for consistent altitude reporting above this transition altitude.
Step-by-step explanation:
After takeoff, the altimeter should remain set to the current local altimeter setting until climbing through 18,000 feet MSL. At that time, the altimeter should be set to 29.92 inches of Mercury (inHg) or 1013.2 millibars (hPa), which is the standard pressure setting known as the standard altimeter setting or standard pressure altitude.
This is because above 18,000 feet, which is the transition altitude in the United States, air traffic control separates aircraft based on flight levels which are determined using this standard setting to ensure consistent altitude reporting regardless of the actual local pressure.