From the assessment called upon each tract of land which has been assessed for the costs of the properties comprising the portion of the project to be used for the generation of electric energy, there shall be deducted by the county treasurer of each affected county an amount equal to such proportion of the total net revenue ascertained to have been derived from the generation of electric energy as the portion of costs assessed under Chapter 2 of this part against the tract bears to the total of such costs. In making the deduction, fractions of cents upon each tract shall be disregarded and no error in the computation of the deduction shall invalidate any such
call.