Fast Formula Input Validation Default Entry

I am trying to create a default value for an element input value, using the number entered into another input value of the same element.

How do I reference the input value that has the amount in it, within the fast formula?

I have tried:
- no INPUTS ARE
- using INPUTS ARE with input_value_name
- using the database item name for the element input value

The formula works if I hardcode the desired input value is - and the default is accordingly created as expected.

Need to know how to reference the input value that has the amount in it, within the fast formula?