The method I ultimately preferred was to look at penalties only as a percentage of rethrows.
Using the example data, this gives 6.06%.
I liked this because penalties cannot come from the initial throw, so using i to determine a penalty Kubb rate seems a little off. Penalties can only come from rethrows, so r seems to be more relevant. A high r could indicate aggressive Kubb throwing, but even with aggressive throwing, penalties are never desired.