Gravity feed ceiling
Flight time above FL 300 less than 30 min - FL 300
Aircraft flight level never exceeded FL 300 - FL 150, or 7 000 ft above takeoff
airport, whichever is higher
If you are on easterly track and flying FL 290 (not crossed FL 300 yet) then ceiling is FL 150 or 7000 ft above takeoff airport.
If on easterly track and flying something like FL 310 (less than 30 min above FL 300) then as ceiling is FL 300. As FL 300 is not available you may go down to FL 290. Here ceiling means the max FL at which gravity feed is possible. No worries if flying below max.