I am looking to make a pie chart that shows workload percentage by task, and if all the available time is not being allocated to these 4 specific tasks, I want the remaining portion of my pie chart to display an "unused time" percentage.

I currently have my pie chart breaking down each task but I am having trouble adding in my section of unused time.

I currently have my pie chart working like this.

I first created an IF formula that looks at what type of task is being performed in the record and it applies the proper time formula.

I am then using this to graph the pie chart;

On change of: Task performed

Show Value: IF formula I just described

This is where I would like to somehow add another sliced for the unused time if the total time worked is less that the total time available.

Any ideas?