Denny O processhead math above checks out, the two “arguments” would be the on why we are assuming the deer burger is 50/50 deer and beef fat. I think it is 80/20 and then what the pork butt starting fat content is. So, if we go with an 80/20 deer burger mix that changes the math, but the equation is correct.
Also, it depends on if the fat cap was scrapped off or not, and how big it was, to begin with. I see pork butts occasionally that are very fatty and sometimes ones that have a minimal fat cap.
From what I see he is mixing 50/50 with straight pork butt and then deer that has had 20% beef fat added. So it is 70/30 (pork butt and being generous with fay%) and 80/20 for the deer burger. So, the fat content of 50% of the mix is 30$ and the other 50% is 20%, making it 25%.
It’s early though, so my brain may be misreading something, however, it was even earlier when processhead responded. Either way, the equation he provides above it correct.