"estimated tax" means the amount which the individual estimates as the amount of income tax imposed by Code Section 4872048-7-20 less the amount which the individual estimates as the sum of credits allowable by law against the tax [...]

"estimated tax" means the amount which the corporation estimates as the amount of income tax imposed by Code Section 4872148-7-21 less the amount which the corporation estimates as the sum of credits allowable by law against the tax [...]