we pretty much use a 70/30 mix for all sausages, and it comes out great. you can get a pretty good idea of the fat content of a butt by looking at the outside. pork has less intermuscular fat, and more intramuscular. most of the fat is going to be visible.
if you're not using a binder or a method of bringing out the proteins to bind it, you need a decent amount of fat to hold the sausage together for free forms.
also, my theory on fat- before you pull out of the flavor, moisture and all the goodness from your meat, you'd do better for yourself to cut out all the simple carbs, processed foods. your simple sugars/carbs dump into your blood at a rate of 30 units per minute, whereas in addition to satiating you (something carbs don't do), fats and proteins can only be transferred to your blood at a rate of 2 units per minute. what does that mean? your cells and liver get slammed full of sugar immediately, the rest gets turned to fat, your sugar drops and you still have a high level of insulin in your blood, and guess what? you're hungry again. you eat less carbs and more fats and proteins, your body can only process it into the sugar your blood can take so quickly, trickling in, giving you a steady, constant sugar level, not overfilling your cells and then your liver, thusly not putting 75% of the food into your fat stores :) this has been over simplified and drama added to emphasize my points, but my main point - before you take out all the good stuff out of your meat, cut down on the pastas, potatoes (chips, fries, mashed, baked, whatever), processed rices, breads, etc. don't cut them all out, but make the main portion of your meal meat/proteins/fats, and you'll get full quicker, stay full longer and feel better! :) (took me 40 years to figure this out, but i've lost 60 pounds in the last 6 months, because of it) :)