Pardon my intrusion. Some stuff I would like to suggest as corrections.
Minimum Focusing Distance
Minimum focusing distance does not depend on focal length. It depends on the dioptre rating; in this case, the Raynox DCR-250 is a +8.
Minimum focusing distance = 1000/dioptre = 12.5cm.
Magnification
Magnification of subject is dependant on focal length of the lens, i.e. 50mm, 85mm, 100mm, 300mm, etc. A 100mm lens with a +8 (Raynox 250) will yield a bigger magnification than a 50mm lens with a +8.
Magnification of subject is also dependant on dioptre rating. A 50mm lens with a +8 will yield a bigger magnification than a 50mm lens with a +4.
Depth Of Field
Depth of field is dependant on focal length AND focusing distance. This doesn't apply to macro only. It's a lens characteristic.
Basically,
the closer you focus,
the thinner the depth of field.
The longer your lens focal length,
the thinner the depth of field.
Here's a very old thread, which is also a Sticky in the Macros gallery.
Here's another very old thread with questions I raised long ago. Perhaps it might be useful, I hope.
=)