Dashboard for Projected Income

I am working on a number of projections and i would like a way to display the difference in 2 fields on a dashboard. In my example i am looking at projected revenue and expense. I can easily have the dashboard display the Sum as seen in the attachment but i would really like another field on the dashboard that shows the difference in these two number.
I have tried a number of different ways to produce this. I created a new board using mirrored fields but i can’t use mirrored fields in formulas so it requires me to manually transpose numbers into a separate field for the calculation.
I have been able to get get something workable by adding all expenses and revenue categories into the same board. It works but would prefer to be able to separate them.