Gift-tax basis clarified: defines tax payable after taxable-gift determinations and treats declared return as basis for interest. Amendments insert explicit cross-references to taxable-gifts determinations, substitute an explanation to exclude additional gift-tax from the phrase 'tax payable on the taxable gifts as determined,' and add an explanation deeming tax payable on assessed taxable gifts to be tax declared in the return for computing interest; they also modify notice language to refer to post-determination assessments and omit the prior Explanation.