Quick table calculation in tableau is really very helpful for doing mathematics or calculation without doing by yourself. So there are many calculations that you can add in your visualization and percentage of total is one of them. Percentage of total helps you to find the percent distribution of a measure along with any dimension. For example if you want to analyze the percentage of sales based on different region from you default sample super store data set, so just drag and drop sales into column shelf and region to rows shelf. After that right click on sales and select quick table calculation. In quick table calculation select percentage of total that’s it, now you will see all the regions are broken down by percentage of sales. Even you can broke the region based on category also, now the overall visualization will look like this.
To learn more about percentage calculation in tableau check tableau training