The problem with that is that he wants to obtain that result from the start because that's what he said and wants to be right, and in a picture things can vary a lot. IMHO he takes too much pixels for the G92b and too few pixels for the G92. By my measures based in that same picture the result is 242mm^2. As you can see things can change a lot.
Another thing that I noticed is that the chips are not 100% square and that while G92 is taller than fatter, G92b is wider than taller. So those who meassure one dimension and just multiplicate it, obtain a very different result depending on which dimension they choosed to measure.
Anyway, I have a 9800GTX+ at home, my brother's card, but he doesn't let me take out off his PC, much less removing the heatsink. I'm still trying, but I've been trying to take a look at that damn card since November, with no luck.