I sometimes use Pinhole Designer (a free download) with a user constant of 1.5 for determining pinhole diameter for maximum on-axis resolution. My tests confirm that there is a rather sharp increase in resolution at the point where the diffraction and geometric resolution curves intersect. A slightly larger pinhole improves off-axis resolution with a loss of on-axis resolution. This may be desirable in some pinhole photography. The below charts graphically show the effect of increasing pinhole diameter. I donated the improvised camera to Eric Renner at Pinhole Resource many years ago, and don't intend to refine the off-axis test. It would be much easier to refine the on-axis tests with today's digital cameras. Maybe someday . . .