I've read about the capabilities of the Set Global Variable action but I can't find it. Where should I look?
Open RM and then scroll where you would create a new rule. There is a bar you click to create global variables, below create new rule.
You can create them and set them in that area. There is a column in the chart that says set, click it to set a new value.
I see where to create them and set initial values, but I've seen other posts referring to a Set Global Values action that includes options for math functions...
Do you mean setting them up as a device? That is a global connector.
I think this is what you are looking for.
If the variable is a number you can select the variable math option here.