In order to determine the correct height for your beam you should first install your ledger board and support posts.  Next you will run a string from the top of the ledger board to the support post with a level and mark the position for the top of the beam.  You will then need to subtract the thickness of the beam and post beam connector hardware.  Finally you should adjust your beam height by factoring in a 1/4″ for every 8′ pitch to slope away from the house for drainage.  Mark this height with a chalk line and cut your post for the correct beam height.