Originally posted by JeffJS
Unless I'm seriously mistaken somehow, reversing the lens does not change the focal length of the (single) lens. Only the distance to subject changes. Use the actual focal length of the lens for your SR.
Yes, reversing a lens, it retains the same focal length. But if you look closely at the camera manual, it says that SR is unreliable at close distances. And of course you'll have SR switched off when using a tripod, which is just about mandatory when shooting with the lens reversed. SR works well with normal lenses at normal (non-macro) working distances. It's not a macro butt-saver, alas.