Rather than thinking about a lens with a specific number of millimeters you might be better off thinking in generalizations of length and your current lens use.

Do you like to use short lenses? For what type of work? Would you use a large camera for that work?
Normal lenses? For what? Would you use it?
Long lenses? For what? Would you use it?

For example lets say you like taking candid portraits of the kids playing in the back yard with short lenses now. While that is doable with a large camera, the challenges of chasing the kids with a big camera are significant and your keeper rate will probably be low. Are you really willing or even considering that type of work with a LF camera? If that's all you use short lenses for now then you probably won't need a short lens for LF.