Displaperture 2.1 is out, with improvements to the corner radius slider. There’s a “sticky spot” at the default radius, and you’ll see the radius displayed as a number while you drag the slider. We’ve also made the slider non-linear, so it’s easier to have fine-grained control at the lower (realistic) radius levels.
Use the new-in-2.0 in-app updating to update, or just download a fresh copy from our site.