Hey Don,
If I understand correctly you want to do calculations based off the 'Total Amount' and display that calculation in a separate field on screen?
You can start by adding a TextInput object to the screen and instead of using the TextInput events add the actions to OnScreenLoad events.
It would look like:
OnScreenLoad ~> Retrieve value for Total Amount ~> Use that to complete calculations ~> Update the TextInput object with the final calculations.
Give that a shot and let me know if you need me to elaborate on any of the steps.
-- Mortaza