First, let me start off by saying how incredibly important this feature is. The data so many organizations could have by being able to create custom calculated metrics in Interfaces is untellable.
When creating big numbers (like the Numbers widget in an Interface), you are tired to a specific field, then you must select an operator. For example, you tie that Number widget to a field in one of your tables, for example, “Revenue,” then select “Sum” and it gives you the total. Let’s say you did that same thing for “Profit.”
Then, with those two metrics, there is no way to create a third one based off those that calculates, for example, something like: Profit / Revenue (displayed as a percentage (Margin)).
The problem is that the only workaround to achieve the above (something that is a basic feature in, for example, Google Data Studio), is to build in the Profit, Revenue, and Margin fields at the table level which can fail to work if you are dealing with a metric for which there is no workable operator. For example, you cannot use “Sum” in the settings of a Number widget in an Interface in order to sum up all the “Margin” values in your table. That doesn’t make sense.
Long story short, I would love if, in Interfaces, you could calculate your own metrics. The flexibility this would provide would help people who are using Interfaces for basic financial or other number-based widgets, and could give them the insights they need (like the organization I work for!).