It is a pretty tough task to measure how productive you are (for the sake of better expectations). Assuming you do succeed at measuring your maximum productivity. The you have to measure is your distribution of productivity over time. This again is quite a tough task. A short cut is to just not beat yourself up over your productivity on a given day. Granted, that maybe easier said than done, but maybe that is what the OP should strive for. Or, maybe he is on the path to doing that.