Custom measures allow you to create any possible metric by performing calculations on the current ones. For example, the list above provides us with clicks and impressions. So we can easily calculate click-through rate by dividing clicks by impressions.
How to Create one in Data Studio
1. Go to Data Studio and open the desired report in Edit mode.
2. Click on the component where you would like to see your new custom measure.
3. Choose "Add metric" on the right menu and then on "CREATE FIELD".
5. Name the field accordingly and insert the calculation from the table above.
6. Hit apply, and you are done. The desired measure will calculate and then show up in the component.
Make it Universal!
You probably want to reuse the same custom metric(s) across multiple components. This doesn't mean you have to create a certain field X times to create your custom report.
To make custom measures easy to use across different tables and charts, you can easily insert them in the data set in the Data Studio.
Click "ADD A FIELD" in the left menu.
Now, a similar window has opened up, and you can insert the same formula here. The difference is that after you save it, it will appear in the list of "Available Fields", and you can easily drag & drop the measure into different components.