For a lens set at infinity you would need focal length equivalent of extension to make it give you 1:1. That means a 50mm Lens would need 50mm of extension at Infinity focus to achieve 1:1 magnification. This formula also comes with some other caveats, like the lens has to be symmetric and all that I believe.
Best thing is to shoot a scale at the MFD and see whats the max magnification you can get.
where o and i are object and image distance, respectively, and f is the focal length. As an example, let's look at the 50 f/1.8. The closest focusing distance of this lens is 0.45 m = 450mm. We now get the image distance from
1/450mm + 1/i = 1/50mm,
i = 56.25mm. The magnification factor is i / o = 56.25mm / 450mm = 0.125. Now add the extension tube. i' = i + 25mm = 81.25mm. We find o' from
1/o' + 1/81.25mm = 1/50mm,
o' = 130mm, the new closest focusing distance. And the new magnification factor is i'/o' = 81.25mm / 130mm = 0.625.
The slight differences from the magnification factors given in Canon's Lens Works are due to these lenses being not exactly "thin" lenses.
