When determining how much earned income a non-profit institution can
generate, it should first be divided into mission-related and unrelated
income.  That is, how much income is generated by activities related to
the mission of the organization (e.g educational programs, admissions,
published works)?  How much income is generated by unrelated activities
(e.g parking, restaurants, travel programs)?
For tax purposes, an organization never wants to have more than 1/3 of its
income being generated by unrelated activities.
For example, the Girl Scouts can not have more than 1/3 of their total
income generated by cookie sales, or else they would be at risk of losing
their 501(c)(3) status.
The Foundation Ceneter has many resources available on this subject.

Mary Enright