The reason for my curiosity on this matter is that I have been reading some on close focusing techniques and several writers claim to use a variety of medium telephoto lenses with attachments (such as extension tubes) rather than purpose made macro lenses for largeish subjects (say, flowers). They claim to get results indistinguishable (for most purposes) from purpose built macro lenses of long focal length at a very cheap price.
One such writer claimed to use a 300 mm lens ( The Olympus OM-Zuiko T4 300mm f/4.5) with an extension tube to obtain the reach necessary to shoot close-ups of marsh plants etc not easily accessible and too far for, say, a 50mm or a 100mm or so macro lens.
I simply wondered how much reach such a combination would provide.
Actually, now we've looked into it, it does look "dooable".