A lot of the settings in Gnome 3 are accessible from the command line using 'gsettings', the command you want is probably 'gsettings set org.gnome.power-manager percentage-critical <percentage>'. Here's a quick tutorial to get you started with gsettings:
gsettings list-schemas | grep <search_term>
gsettings list-keys <schema>
gsettings get <schema> <key>
gsettings set <schema> <key>
Have fun exploring ... also, if you stuff up and Gnome won't start you'll need to prefix your commands with 'dbus-launch '.