Install IOS 8 Update without enough space
I tried to install my IOS8 updated on my iPad mini and iPhone and get the error that the must be 45Mb of free for installation to continue.
However I discovered while trying to install certificates on my iPad I could use the Apple Configurator to update the IOS8 and install my certificates with my Mac Book Pro.
Download the Apple Configurator from the App Store and then install it on your Mac.
When you install the Apple Configurator you can plug your iPad mini or iPhone Device in to the USB port.
You will need to go to Settings / iCloud / Find My iPhone and turn that off.
The Apple Configurator will then find the device.
Then you can go to the Prepare Icon on the top bar of the Apple Configurator as shown below:
Once you have plugged in the device you can press the Prepare button at the bottom of the Apple Configurator.
After that you can just wait until the IOS 8 version is downloaded and installed.