How Do You Use The Speed Control Slider On Joystick?

I have a logitech X3D Pro that I picked up at a yard sale new for only $3. Its an awesome joystick. I want to make the slider control the speed of the robot. I can see that when I move the slider up and down, the green dot in the joystick window moves left when I push the slider up and right when I push the slider down. I am trying to write a script to use the slider for the robot speed control. What variable name do I use? What is the range of values for the variable as well? Any pointers on the best way to write an ez script for this? I have attached a screenshot.