How to calculate totals from different forms and display on dashboard

How to calculate totals from different forms and display on dashboard

Hello team, 
I'm currently using the Free license to test and train myself on how to create apps using Zoho Creator.

I have developed a small application to manage our fundraising activities. I have two main forms :
1) Contributions - Records all members contributions
2) Fundraising Activities - Records all our fundraising activities including a subform that records our expenses related to each fundraising.

What I need to calculate :
Now I need to calculate the Total Income to display on my dashboard. My formula is simple :
  1. Total Income = Total Revenue - Total Expenses
  2. Total Revenue = Sum("Contribution Amount") (Contributions form) + Sum("Revenue Received")(Fundraising Activity form)
  3. Total Expenses = Sum("Expense Amount" in subform Expenses in Fundraising Activity form)
Please advice how I can combine the calculations from the different fields to have the "Total Income" value

Thank you
Cheers
Fiona