I don't brine pork butts, only poultry.
For the brine to really make a difference in the flavor of the pork you would have to leave it in the brine for a long time.
You might think about injecting, or just using a finishing sauce, of which you will find several on this site.
Here is mine:
3 Quarts of Apple Cider Vinegar
1 cup Ketchup (Catsup)
5 oz. Worsty sauce
10 oz. French's yellow mustard
3/4 cup salt (I use sea salt but regular iodized is fine) - I also usually cut back on the salt
1/4 cup fresh ground black pepper
1/2 oz crushed red pepper
4 oz of your favorite hot sauce (I cut back on the red pepper and hot sauce depending on the audience and it doesn't affect flavor)
Mix all ingredients in a large pot and bring it to a boil. Once it boils it is ready for use as a mop or for canning.
Makes about 1 gallon, which is enough to mop a 70 pound pig.