i use pork butt for general grind and sausages. i use bacon for grind i know im gonna press into hamburgers. for general grind i go 30% fat. sausage recipes vary widely so i follow the recipe. burgers i only do 20% bacon otherwise it seems to overpower.
if you've got a recipe you know you love that uses ground - say a meatloaf - make a couple batches at different ratios and see which you prefer. a lot of this you'll tweak/change as you go along.