OK, thanks for trying, everyone!

I have been busy too, and found a graph in a book (SLR Photographers Handbook, HP Books, Tucson AZ, USA, 1977) written by Carl Shipman (a very informative book, by the way and, unlike many books these days, without spelling or grammatical errors) which provides the "Lens to Subject Distance for Magnification by Extension.

According to this (non-linear) graph, the answers to my questions are as follows:

In the case of the 100mm lens: approximately 45cm. (Prev.:1m)

In the case of the 200mm lens: approx. 1m. (Prev.: 2m)

In the case of the 300mm lens: approx 2.1m. (Prev.: 4m)

Thanks all.