
In the equation, e(t) is the percentage excess return in month t. r is your portfolio return, m is the market percent return given by your benchmark index, and f is the risk free rate. The normal return is a mix of the market return and the risk free return weighted by beta. I use the MSCI index as my benchmark. Specifically, the All Country Gross Index. Make sure when you choose a benchmark to include dividends. The net and gross MSCI indices do include dividends - the gross is pre-tax and the net post-tax. Total return data for the S&P 500 can be found here. I use the 90-day Treasury Bill rate as the risk free rate (you'll need to compute the monthly rate from this annual rate).
You can estimate beta in more or less sophisticated ways. If you are 100% in stocks and guess your stocks are average and you are using no leverage, you can use a default of one. In this case, the excess return is just your return minus the market return. A more sophisticated approach is to compute the weighted average of the betas of all your stocks and funds, which you can find on Yahoo Finance for example. More sophisticated still is doing some kind of regression analysis - you need a track record of your monthly returns to do that. My preferred method is a time series model that allows my alpha and beta to vary over time.
To find how much your actual excess income is per month, you then need to multiply the percentage excess return by your net worth. I've done it here, using the S&P 500 as the benchmark:

The bars are the monthly "risk-adjusted excess incomes (or losses)" while the light green line is the total of the last 12 months. A strong cycle is very clear - I've gone through periods of above and below par performance fairly regularly. Alpha smoothes all this out into an estimate of the average excess return. The following chart does this using the MSCI instead as the benchmark:

There is no guarantee though that this level of performance can be maintained. The huge fluctuations on the SPX chart above make that clear. In this post, I was wondering out loud whether I could maintain that performance in the coming year by looking at where the returns would come from. I wasn't saying I could maintain that indefinitely. I don't know.