For a good sausage I work to a ratio of around 80:20 meat : fat which can be adjusted according to taste. A mix of pork shoulder and pork belly (2:1 or 1:1) may be used but after making sausage for a number of years I have settled on pork collar (pork neck in some countries?) and pork belly in a 2:1 or 1:1 ratio depending on the sausage I am making.
Typically I grind meat for sausages twice but have found that a single grind produces a really nice coarse mix ideal for some sausages such as a fennel Sicilian pork sausage.
As a caveat if you are mixing lean beef and pork 60:40 as a ballpark figure pork shoulder/collar is around 15-20% fat and belly 30-35% fat therefore you will need to adjust the pork fat content so the end product is around 80% meat 20% fat, sometimes a bit of trial and error is required particularly as you want a juicy end product but that is part of the fun!
The fat content figures is for UK pork, not sure if US or Canadian pork differs? It is always worth talking to your butcher if you get a chance!
