Pinhole Designer defaults to the Lord Rayleigh user constant. What Lord Rayleigh derived from theoretical calculations 120 years ago differs somewhat from what many pinhole photographers have derived from experience since then. For better central sharpness I prefer a user constant of 1.4, while others choose a slightly different figure. We can try to make a science or an art of pinhole photography. Trying too hard to perfect both in pinhole photography is counterproductive.
The poor man's answer to the Skink system is matboard mounted pinholes secured in screw-in filter rings with the glass removed.