Whenever any tax required to be paid by this chapter is not paid in full at the time required for such payment, it shall be the duty of the Secretary, subject to the limitations prescribed in section
6501, on proof satisfactory to him, to determine the amount of tax which has been omitted to be paid, and to make an assessment therefor against the person liable for the tax. The tax so assessed shall be in addition to the penalties imposed by law for failure to pay such tax when required. Except in cases where delay may jeopardize collection of the tax, or where the amount is nominal or the result of an evident mathematical error, no such assessment shall be made until and after the person liable for the tax has been afforded reasonable notice and opportunity to show cause, in writing, against such assessment.