This is right, because it's exactly what one does when one uses a regular color film with a pinhole camera (and does not notice any sharpness difference in different colors). And moreover, one has no concerns either when using B&W panchromatic film.
I will just choose a mid point wavelength, say 550.
This I don't quite understand : isn't your camera designed to have the same optical paths in the three colors ? if it's so with a lens (to get three views of the same size and same focusing) then it's so also with a pinhole ?
Then,, each film plain will be a diffrent distance from the single pinhole, so I will choose the longest for exposure calculation
isn't your camera designed to have the same optical paths in the three colors ?
Of course, you correct. The diagram attached to my opening post in this thread clearly shows this,as dose the camera itself. So I am clearly working with one distance. If I eliminate consideration of 3 different wavelenghts I need only consider a pinhole of optimum diameter for 12 inches. This is the distance from the lensboard to film planes with bellows totaly contracted. But I will also get some pinhole sizes acurate fore some bellows extention.
I have decided to get some specialy prepared pinhole for this project. And to not use readily availible laser-drilled ones. I wish to do everything right for this project.