I would do "post tax", so you are capturing the amount you save that is actually available for saving in the first place. Granted 401k, etc. can alter your taxes.
But, does it really matter specifically how you do it as long as it is reasonable? The goal of knowing this is so you can adjust if go below (or get ahead). i.e. if you save 30% under one measure, but 20% under another measure, it is still the same amount of savings.
My suggestion is to choose a metric and stick with it. And, if you are comparing your savings rate to "guidelines", then simply know how the guideline defines savings rate.