They're called Expressions. You can find a list of the built-in System Expressions.
If you have two global variables 'a' and 'b' and want to know what percentage of 'b' is 'a', just use:
global('a') / global('b')
This will give you 0.5 for 50% (which is mathematically correct). If you want it as a scale up to 100 for 100% just multiply by 100:
(global('a') * 100) / global('b')
To create an event that runs when 'a' is under 50% of 'b', use System -> Compare Values to create:
(global('a') * 100) / global('b') is less than 50
This will run every tick when it's true, so if you play a warning sound, it'll keep playing all the time. To prevent that, add a 'Trigger Once' condition beneath it, then the actions (playing a sound or whatever) will run once the first time the percentage drops below 50.