Dee - I have a recipe that I use with my rub, but it would work with anybody's rub:
8oz. Apple Juice
2-3tbs Soy Sauce ( to taste )
2-4tbs of the rub that you are putting on the outside of the butt. ( Depending on salt content of the rub )
Put these ingredients in a small stew pot and heat only enough to dissolve the rub ingredients. Then, chill this liquid and strain out the solids so that you can inject without plugging your needle.
The resultant liquid should taste like a au jus from the meat, plus a little taste of the rub.
T