you were originally happy with the method until I pointed out that your answer was doubly-exaggerated due to not one but TWO errors on your part
No. My very first post in this thread was that there were two ways to calculate GM and the big issue was that they didn't agree. See #msg-75194420. And I was happy only because, via my math error, I thought they had been brought into alignment. But as per your correction, they are still not in alignment - and thus I'd pick an estimate in the middle (around 50%).