Of course, you can create Calculated Fields based on a calculation between different fields (such as the Profit Ratio). However, the really interesting aspect of Calculated Fields is the functions. There are many different functions, some basic, some hard to understand. Each function returns a specific data type, and some require arguments. 

Tableau has made it easy: each function has a clear description and examples for learning how to use them.

Going through all the functions and repeating the description and example already available in Tableau doesn't add any value. My strong advice for you is to take fifteen minutes to look at all the functions to have a clear overview of what you can and can't do. 

Here's an example for the round function:

In the following sections and chapters, we'll often use calculations, so don't be afraid, you'll practice them. Speaking of practice, it's time for a guided tutorial!