Measuring tension in double butted spokes
Tensiometers test deflection and their charts make the assumption that the entire section being used for the test (the entire length of spoke between the tool's two contact points) is a uniform thickness. That way that section is isolated - if there were any of the taper in there, it would throw off the reading. So you always make sure the tool is touching only the thin section, and you use the chart entry for the thin section.