The Profitability tab gives you a breakdown of what exactly goes into determining the Net amount shown on the Details screen:
You will see at the top the amount you have set up for your default Sales Tax and default Discount percentage from the Settings screen.
To edit them for this product, tap directly on the number displayed
You will be taken to a new Update screen
Tap on the number you would like to edit
Type in the amount you would like to change to
Tap Save Changes
Next, you will see the *List Price* of your product, the estimated Amazon fees, the Cost/Unit you provided, the estimated Inbound Shipping cost to FBA based on your entry on the Settings screen, and then the estimated Net Profit you can expect to make for both FBA and MFN channels
These amounts will change automatically if you alter areas such as Cost/Unit, List Price, default Shipping cost, etc.