I'm with Rezed on the 20% fat. I know that pork shoulder/butt is generally assumed to be 20% fat, and when I make sausage, I chuck the whole thing in there and the fat level comes out to my liking. I'd go with
480g wild hog (Class I)
400g pork butt
120g back fat
Also, in my extremely short career as a sausage maker, I think that adding water to the sausage is very important. I add anywhere from 3% - 10%, depending on what I'm making. My theory is that the water is bound by the myosin/actin in much the same way that jell-o gelatin binds water when you make jello. I also feel that with the water binding with the proteins, you're essentially making a meat jello (yum!) that will make the sausage lighter and keep it from a dry mouthfeel. Like jello, it's reluctant to let that water go. It all binds together into a sausage shaped terrine, so the amount of mixing is also important to get enough proteins to join the party and keep everything together. I aim for "pretty sticky" and haven't had a crumbly sausage yet. I've also read here that too much fat can make for a crumbly sausage too.
Of course, I may be all wet, but I'm sure someone will let me know if I am!
I posted a recipe for Butterbean when he bagged some wild hog and beaver. He seemed to like it and also came up with some tasty additions to it. Thread is here:
http://wedlinydomowe.pl/en/viewtopic.php?p=36463#36463
