Portfolio Return Calculation Methods

John is analyzing the performance of his investment portfolio over the last year. At the beginning of the year, the portfolio had a value of $100,000. Throughout the year, John made an additional investment of $20,000. The portfolio's ending value at the end of the year was $130,000.

To assess the portfolio's performance correctly, he needs to calculate the rate of return, which should be adjusted for the cash flows during the year. What is the appropriate return calculation method John should use to determine the effective rate of return on his portfolio?