The task is to find the percentage of total sales by region. There are two ways to do this in Tableau.
Method 1 (LOD Expression):
Step 1: Create a new calculated field in Tableau called "All Sales" and type
{SUM([Sales])}
Step 2: Create a new calculated field called "% of Total" and type
[Sales]/[All Sales]
Step 3: Right click "% of Total" and select Default Properties -> Number Format -> Percentage
Step 4: Drag "Region" to rows and "% of Total" to rows
Step 5: Sort Order Descending and Sort By Field "Sales" with Aggregration as Sum
Method 2 (Quick Table Calc):
Step 1:Drag "Region" and "Sales" to Rows
Step 2:Right click on "Sales" and select Quick Table Calculation -> Percentage of Total
Step 3: Sort Order Descending and Sort By Field "Sales" with Aggregration as Sum