(a) In general. The declaration of estimated tax by a corporation shall be made on Form 1120-ES. For the purpose of making the declaration, the estimated tax should be based upon the amount of gross income which the taxpayer can reasonably be expected to receive or accrue as the case may be, depending upon the method of accounting upon the basis of which the taxable income is computed, and the amount of the estimated allowable deductions and credits to be taken into account. Such amounts of gross income, deductions, and credits should be determined upon the basis of facts and circumstances existing as at the time prescribed for the filing of the declaration as well as those reasonably to be anticipated for the taxable year.

(b) Use of prescribed form. Copies of Form 1120-ES will so far as possible be furnished taxpayers by district directors. A taxpayer will not be excused from making a declaration, however, by the fact that no form has been furnished. Taxpayers not supplied with the proper form should make application therefor to the district director in ample time to have their declarations prepared, verified, and filed with the district director on or before the date prescribed for filing the declaration. If the prescribed form is not available a statement disclosing the estimated income tax after the exemption and the credits, if any, should be filed as a tentative declaration within the prescribed time, accompanied by the payment of the required installment. Such tentative declaration should be supplemented, without unnecessary delay, by a declaration made on the proper form.