I think overclock should be calculated from whatever the stock clocks of the card you are using are. That is the true overclock you achieved. Whatever overclocking done by the factory should not count as that card is sold and garanteed to run at this higher speed out of the box.
BTW is there any way to raise the memory clock limit? I have afterburner maxed out on memory with no issues. I would like to try and push it up a bit.