Billable Time Spent

Within Projects, I'm rolling up Effort and Time Spent from all subtasks. With those totals on the project level, I made a custom field to show Time Spent % which is simply [Time Spent]/[Effort].

I've quickly come to the realization that rolling up Time Spent will include time from both Billable and Non-Billable tasks.

My team needs a custom field which would be Billable Time Spent. I could then take Billable Time Spent and divide by Effort to have Billable Time Spent %. This metric is a general pulse check on how we are pacing in terms of burning Effort.

I'd like to be able to accomplish this via Custom Field in the Table view of a project, instead of creating a report.

Is this possible now or on the roadmap for the future?