What is the correct distance or focus to use when using this extra lens??
It is to project an image of the subject into infinity (thus yielding parallel rays). The taking lens is set to infinity and then a sharp image is projected onto the film, when the subject is at the focal length of the close-up lens.
This is the concept behind close-up lenses.
The focal distance typically is indicated on such lens by means of the unit diopter (reverse of focal length in meters), sometimes in coding or maybe even not at all as seemingly in your case.
To then establish its focal length the easiest way is to project an image of the sun as a sharp point onto something heat-resistant. The necessary distance is the focal length of the close-up lens.
When setting the camera at shorter focusing distances with a close-up lens mounted, the focusing distances for the close-up lens become shorter. You can find them at tables.
The focusing distance for a close-up lens is always related to the position of the close-up lens, not to the film!
(Unless explicetely stated otherwise.)
With this in mind you should be able to use your Agfa lens without even further means.