I completely see what your saying with the problem of using “take home pay” to calculate the savings rate. I think one of the most debated issues in the “FIRE community” is finding a universally accepted definition of a “savings rate”.
I agree with you, we have to take into account contributions to 401K (or RRSP’s in Canada). Me and my wife both have defined pension plans at work. Those pensions will play a massive role in our retirement income, but are difficult to fit into the “take home pay” savings rate.
I think I’ll be writing a post soon exploring this issue and the different ways to track your savings rate.