I can also agree with setting the default to 100%, especially as I was also confused by this when first testing the feature. I would think most people with workflows featuring screentones would prefer pure duotone.
Also relating to the screentone generator, there is a minor inconsistency between the defaults of this Screentone Fill and the reworked Halftone Filter implemented in the nightly builds: While the Screentone Fill has a default rotation of 0 degrees, the reworked Halftone Filter has a default rotation of 45 degrees. Not sure if this is intentional, though it might be since the default rotation of the current Halftone Filter is also 45 degrees.