How does the supervisor estimate the greeting (tahiyya) and food (tu'ma) from the the endowment (waqf) if the endower (waqif) did not specify their quantities and then died?

This is determined by custom, as reference is made to the usage of the word "tumah" (food) in those times. It is most probable that they intended it to mean food in Ramadan (definition), such as breaking the fast and dinner for those who are fasting, even if it was for three or five needy individuals who required food to break their fast.