If you need to have a specific information displayed on your data tables, you can choose to build Custom Columns, available in "Customization" on the top right:
From here, you can build your Custom Column:
Creating the custom column is very simple: simply click on "New custom column", and you'll open the following window, allowing your to design your addition.
As usual, start by giving a name to your column to find it again in a second.
In the type section you can choose between a few options, and you'll need to choose wisely as this is what your column will be displaying. The Formula you'll design afterwards will be based off of this too.
This is where you can build from scratch your column.
Start by choosing the metric on the left, from the drop down list:
From there, either choose a metric, or set a specific value on which you want to base your report on. Then you're free to build the rest of the formula however you wish: add an operator and another metric at the end, and even add another operation afterwards if you want to.
AS you're building the column, you'll get an idea of how the stat is looking overtime, as we'll build a chart with it to give you a preview!
Be careful as you can only edit Custom Columns, so try to recycle and use old and un-used ones to create new ones!