I use dry milk powder in sausage because for me it makes much juicier sausage. Not being experienced at sausage making, my first few attempts were disappointing at best. While the flavor was good, the cooking process would squeeze all the moisture out leaving me with dry, mealy, crumbly sausage. This would happen no matter what method I used for cooking them, poaching, pan frying and even smoking at 225˚. I read about using the milk powder as a binder, tried a cup to a 5# batch and have used it ever since. It made a HUGE difference in the texture.
The ratio I've settled on is very simple and works for whatever sausage I'm making:
5lbs pork butt
a 1lb. package of fatback, from which I remove the skin, leaving probably 12oz.
1 cup dry milk powder
Whatever seasonings the recipe calls for.
My guess is this makes for a 25%-30% fat to lean ratio and while not exactly diet friendly, tastes pretty good. By the way, I've only made fresh sausages, haven't tried anything cured or dried.