I wrote
Basically all of the shoulder isn't meat and all of the fat isn't fat. For a lean pork shoulder a figure of 8.8 % fat is used and for hard pork back fat a figure of 78.6 % fat is used. Therefore your fat content is (1.5 x 8.8/100) + (0.5 x 78.6/100) = 0.525 lbs of fat. So 0.525/2.0 x 100 =26.25 % total fat content.
You wrote
When Parsons Snow did the calculation using my pork mix figures he came up with around 25%. I am using a different estimate for VL trim, and it comes out to 40%. If I used 25% based on my method my sausage would come out like sawdust. So I believe we are using the same pork mix you call 25%.
You wrote
I use a pork shoulder with outside fat trimmed so I can get the weight of "lean" pork, which I take to be 20% fat. Then I use 1.5 lb lean to 0.5 lb fat, giving a 40% fat.
This is not �lean� pork but 80/20 (also referred to as 80 % VL, 80:20 visual lean) ie 80 % lean meat and 20 % fat.
The recommended/typical fat content for 80 % VL Pork Shoulder/Boston Butt is 28.0 %
The recommended/typical fat content for hard pork fat back is 78.6 %
Therefore using the above figures your fat content is (1.5 x 28.0/100) + (0.5 x 78.6/100) = 0.420 + 0.393 = 0.813 lbs of fat.
So 0.813/2.0 x 100 = 40.65 % total fat content.
I hope that this clears everything up.
Kind regards
Parson Snows