Nice to meet you. You are not very far off. This is called a compounded return. It assumes that you invest $1 in the 1st period and if it grows 20% it becomes $1.20, then you take $1.20 and invest for another period and so on. If you need to know the final result you just multiply the numbers,however it assumes that your entry price is equal to the closing price in the last period. However, for month to month return you can observe gaps, so the grows rates need to be adjusted. If you add all you return you just make an assumption that each period you invest your $1 in the begining and the rest goes to you pocket.