The board shall at any of the following times estimate the amount of money it is necessary to raise for the purposes for which construction bonds may be issued:
(a) As soon after the district has been formed as may be practicable.
(b) Whenever the board finds that the construction fund raised by the next preceding bond issue is insufficient or has been exhausted by expenditures therefrom and it is necessary to raise additional money for the purposes for which construction bonds may be authorized.