Section 9 of ASTM B267 covers the test for nominal electrical resistance per unit length for resistance wire. The nominal resistance per unit length for round wire is calculated from the nominal resistivity and the nominal cross-sectional area.
For ribbon or flat wire produced by rolling from round wire, the cross section departs from a true rectangle. The conventional formula considers the cross section 7% less than a true rectangle when width is more than 15 times the thickness.
Ribbon is actually made to its specified resistance per foot, and no tolerance is specified for thickness, allowing the manufacturer to achieve the target electrical resistance regardless of minor geometric variations.