Tableau has built in "percent difference" calcs, so I don't believe you need to do this. If you'll post a packaged workbook (.twbx file), someone here can set that up for you and show you how to utilize it for your data.
You can use the sample data provided to mock up a scenario, if your data is sensitive. Cheers
If you attach a .twbx file someone on the forum could create the calculated field for you. Just go to File - Save As and change the "Save as type" to a packaged workbook and then upload it.
If you don't want to do that then check out this link from the On Demand Training and let us know some more about your data. For example, is Quarter the aggregate of Months, Days or is there literally a row in your data set for Quarter 1 Year 1. Those types of things will matter when answering this question.
Actually, I'll do the work for you. See the attached. I simply right clicked on the Sum(Sales) pill, and chose "Quick Table Calculation" > "Percent Difference". In this view, it defaulted to the correct setup I wanted, but if your dataset is more complex, it can be modified to compute in a nearly infinite number of ways.
Here's a quick tutorial: Calculating Percent Difference Across Dimensions | Tableau Software
Welcome to the Forum! This is a great place to learn.