776.1.19. In this Title, “unused portion of the tax credit” of a corporation for a taxation year means the amount by which the maximum amount that the corporation could deduct under section 776.1.20 for the taxation year if it had sufficient tax payable under this Part for the taxation year exceeds the tax payable by the corporation for the taxation year under this Part, determined before the application of that section and the second paragraph of section 776.1.21.