I don't count finishing position when a driver is out or loses positions because of a mechanical failure, pit stop error (like not putting on a wheel) or crash not caused by them. It's still far from a perfect science b/c the Merc driver, especially Lewis, are hurt b/c of the race pace of the car. I take the average qualifying position and average finishing position to get the +/- positions gained or lost. I throw in their average points scored per race and that's pretty much how I get these calculations. The average positions gained/lost is killing Lewis right now. To be honest though, this is a pretty good metric, it's just that the tires are destroying my theory! Maybe that will change in Hungary?
I'm working on figuring out another metric or two to add in for 2014 so it is a little more driver friendly.