The highest resolution in my example above was indeed about 5.5 lp/mm on-axis with the 3.5 inch focal length. Changing the focal length changes the optimum pinhole diameter and thus changes the resolution. A contact print from a short focal length pinhole camera can appear quite sharp at normal viewing distances, but we tend to look at such small images at a closer distance. An image from a long focal length pinhole can look quite sharp if viewed from the distance which provides correct perspective. One way to predict this is to look at the actual backlit pinhole at a distance equal to the image viewing distance. If the pinhole appears to be a disk, the image probably won't appear to be sharp. The longest pinhole focal length I've used was about 24 feet for a solar eclipse. The negatives were dismally unsharp at arm's length, but sharp enough at a viewing distance of many feet.
Sharpness can be clinically measured with resolution charts, but other subject matter imposes other standards of sharpness. A subject that is more shape and shadow than detail may not require nearly the resolution that a subject with much fine detail does. Image size and personal preferences are even more important in determining the required sharpness.