Net wealth determination clarified: tax payable treated as declared in the return for interest computation under amended provisions. Amendments to section 17B expand the reference for net-wealth determination to include net wealth determined under section 16(1) or on regular assessment, substitute the Explanation to exclude additional wealth-tax from 'tax payable' under section 16, and add a deeming rule that, for computing interest under section 15B, tax payable as determined or on regular assessment shall be treated as tax payable on the net wealth declared in the return.