Zane, to add to what Ryan has provided above...
You'll have a process variable as well as a task variable. When you create the task variable you can configure to initialize from and copy into the process variable. This allows a value to be passed from one workflow node to another.
Ryan - I like using separate Expression Robots (as you have done) so I don't lose track of code. The code could also be placed in a Task Transition such as Complete.