Found a post on reddit: Someone is looking to make the legend in Tableau centered. Centering the legend is not a feature in Tableau desktop and a limitation in Tableau, but there’s a workaround that is effective. 
The objective is centering Casual and Member under the pie chart. We can use Horizontal Container Objects in Tableau dashboards to center the legend.
- Drag and drop horizontal container
- Add two blanks to the container
- Add legend in between
Lastly, we can distribute the contents evenly. This is a setting inside of the container. However notice this hides two things in our legend and may not be what you need, but at least you’re moving the right direction and have two options. 
Using one of two options above will get you close to where you need to be. Next you may want to start playing with the legend sizes in the legend by highlighting halfway point between two dimensions. 
Alternatively you can stop trying to do things that Tableau can’t do, and reconsider an alternative approach that doesn’t generate workarounds that are faulty to breaking. For example, when they swap to mobile, or ipad, what happens to your dashboard here? Are you accounting for changes at these levels today? During my time tableau consulting I’ve found people prefer legends on the right or left side of the chart VS below the chart, however if the designers asking for it, above is a few approaches.
You may try floating your legend VS fixed sizing.
Floating has a few oddities when resizing the dashboard, but for quick wins you can’t go wrong with a floated object.
Side view of legend. Alternative legend view?
I understand side view VS center isn’t the original question, but for documentation sake, let’s look at this screenshot above a little more. Notice you’re in a similar boat as the previous setting. This is a part of Tableau that you’ll need to work around using containers and blanks, or you can use “padding”… This side view scales well when more dimensions are added, and easy to fix.
Using Padding of 200 pixels on TOP we can push down the legend down VS using the previous BLANK in a container.
Padding to push the legend around may be best. I find myself using PADDING more than floating blanks.
Here’s the example of how padding can be used for the centering too.
Reddit blog.