A private pilot correctly sets her altimeter to reflect a reported altimeter setting of 1012mb at her departure airport. She then climbs to an indicated cruising altitude of 1000m above sea level. The highest terrain elevation between her and her destination is 850m above sea level, which yields a fairly comfortable margin of 150m. However, she does not realize that she is flying into an approaching low pressure system and that the correct altimeter setting will decrease to 992mb well before she reaches her destination.
a) If she fails to update her altimeter, what will her actual altimeter margin be in the worst case as she crosses the highest terrain?
b) Explain why you think that the standard procedures require altimeter settings to always be rounded down.