Mac App Store update stuck (Installing - Calculating...)
In an act of desperation, I managed to fix the issue by:


Deleting /Library/Updates
Doing 

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist RecommendedUpdates
sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist DidRegisterLocalUpdates


in a Terminal session
Restarting the Mac


The update is re-downloading now.

Interestingly, before I tested these steps, I found the update ready to install in /Library/Updates, however when I tried to install the PKG file, Installer claimed that another installation needed to finish (?). Deleting it and restarting fixed the problem.