At what altitude must you be on oxygen if leaving the cockpit and the flight attendant is in your seat?

The correct altitude for requiring supplemental oxygen when leaving the cockpit, particularly if the flight attendant is occupying the pilot's seat, is indeed above FL250. At altitudes above this level, the partial pressure of oxygen decreases significantly, which can adversely affect cognitive functions and lead to hypoxia if individuals are not using supplemental oxygen.

When flying above FL250, pilots are generally required to use oxygen systems in certain circumstances, especially during periods where cabin depressurization may occur or when not at the controls of the aircraft. This altitude is critical in ensuring that safety standards are met to maintain cognitive function and the ability to perform necessary duties in the unlikely event of a cabin decompression.

Options at lower altitudes, such as FL180 or FL300, do not require the same strict guidelines for supplemental oxygen for every situation, hence the selection of FL250 as the relevant threshold for the described scenario.