This will be a post in progress, as I'm still working out the nuts and bolts of this. I've been interested in close-up photography with my Holga and Diana cameras, and I've been gathering information on how this works.
First a little information. On any camera, you can shoot close-up photos by attaching a close-up filter to the front of your lens. These close-up filters strength are expressed in numbers called diopters.
The formula for determining how close you can focus, is:
focus distance (in meters) = 1/ diopter
On a Holga, screw-in diopter filters may be attached by using a step-up ring. If you are a 35mm photographer, you may already have a set of close-up diopters you can use. If you have 52mm filters, get a 46mm-52mm step-up ring. If you have 49mm filters, get a 46mm-49mm step up ring. The 46mm end of the ring can be screwed into the plastic of the Holga lens. The diopters can now be attached to your camera.
For a Diana, I am still researching and experimenting with ways of attaching the close-up filters. For now, I would suggest tape. If you have a more elegant solution, please let me know.
I haven't checked the chart to see how accurate it is. That is the next step. I will post results when I have them.