When computing my savings rate, I don't like to just add pre tax and post tax investments, since the former is subject to taxation on withdrawal. I prefer a metric that regards pre and post tax saving at some exchange rate, cognizant of the fact that a saved post tax dollar is worth more than a saved pre tax dollar.

In my own computation, I fix this by computing a tax liability on the 401k savings, and subtracting it from both the numerator (amount saved) and the denominator (amount earned after cost of income).