Lord Rayleigh approached pinhole photography more as a scientist than as a photographer. Photographers often arrive at slightly different formulae based on experience. For the best resolution of a test chart in the center of the photograph, I use PinholeDesigner with a user constant of 1.4 for panchromatic film. Yes, the color of the light source and the color response of the film does slightly affect the optimum pinhole diameter. I'd use a 0.33mm pinhole in Blindpig's camera. Also, the pinhole diameter can be slightly reduced for close-up photography. Since the ultimate goal of pinhole photography to most photographers is the photograph, not the validation of scientific theories, experimentation is in order. We each have our individual tastes.