How to Use Formula on Another Formula-derived Field

How to Use Formula on Another Formula-derived Field

I'm trying to calculate a value based on another calculation.

Example:

Pay1 + Pay2 = Total Pay, where, Pay1 and Pay2 are fields in the form and Total Pay is a formula field.

Total_Pay / Hours = Wages, where Total_Pay is the formula field above and Wages is another field.

When I attempt this, I am given an error saying the variable Total_Pay is undefined.

What is the newb doing wrong?