The thin lens equation gives an estimate 1/d + 1/x = 1/f, where d is the distance from the lens to the sensor, f is the focal length of the lens, and then x is the distance from the lens to the object in focus. At infinity focus, d = f. Now you're adding an extender, so d = f+(extender length), and so we get "infinity" focus distances of 3.72/3.99 meters for the 18G and 1.64/1.93 meters for the 45G. The two numbers are distance from lens and distance from sensor, respectively.
Of course, the 250 is NOT a thin lens, so these are all approximate. All the supplied data suggests that closest focus corresponds to a lens extension of ~50mm, so that's some comfort.
--Matt