I do not fill the oil to the max mark. Halfway between the min and max seems to be the optimal. If you constantly fill to the max mark, you may find that your oil consumption goes way up. Mercedes Technical Service Bulletin 00/57A dated Nov. '95 talked about this.
And I find that the best time to check the oil is in the morning before you start the car. You will have a consistant base point this way....oil level can change dramatically depending on oil temp and water content in the oil.
However you decide to check your oil, I think doing it in a consistant manor will give you a better indication of your consumption.
Also, sometimes that sensor (oil level) starts to go bonkers. Some have luck cleaning it, some have luck changing to a synthetic (although most oils now-a-days keep your engine internals pretty darn clean), some people have to replace the sensor.