. Your T-Rex Score is the percentage of an investment return you get to keep after fund fees have been subtracted; that is T-Rex Score = Net Return on Investment After Fees Total Return on Investment This is typically calculated over a 25 year period, with yearly compounding interest. Common mutual funds have Management Expense Ratios (MERs, also known as fund fees) of 2%. Suppose that you realize an effective annual return of 8% every year. What is your T-Rex Score? Reminder: Your return on a single principal investment is the difference between your final amount and your invested amount. So if you invest $100 and finish with $120, then your return was $120 $100 = $20