This is an extension to idea A-I-13913.
Ideally, if a column can be added to a list report then it should be filterable and available to include in calculation columns. A good example of where this is missing is the workspace settings report which doesn't allow users to perform filtering on any of the workspace settings columns. Nor can they be included in calculations.
There's always a sense of disappointment when trying to write a formula and finding the field you need isn't available. My particular use case was trying to write a formula based on custom tables to try and visualise the % of fields showing a shared value, something I've been working with Stuart Blair on your team