A grantee shall calculate the amount of endowment fund income that it may withdraw and spend at a particular time as follows:

(a) On each date that the grantee plans a withdrawal of income, it must—

(1) Determine the value of endowment fund income by subtracting the endowment fund corpus from the current total value of the endowment fund on that date; and

(2) Calculate the amount of endowment fund income previously withdrawn from the endowment fund.

(b) If the value of endowment fund income in the endowment fund exceeds the aggregate amount of previously withdrawn endowment fund income, the grantee may withdraw and spend up to 50 percent of that excess fund income.