How Do I Define A Global Real Variable In Aspen Plus?

Hi there, I'm looking to make a flexible simulation so I need to figure out how to define a global real variable.

I want to be able to use this variable in places such as using a global variable to hold the value of the total molar flowrate of a stream going into a distillation column. I can then use this variable in such places as "distillate rate" when specifying distillation column operating conditions or specifying the bounds on a manipulated variable (such as the "varied" variable in a design spec).

How can I declare a usable global variable?

How do I do math on a declared global variable? Do I do it directly in the input field its used in?

Thanks a lot guys, I appreciate it.