Main Vent help Please

Pat_17

Can someone please give me the standard for venting the main. How many Gorton's # 1 would you use per foot? My main is 55 feet of 2 in piping.I currectly have a Gorton's #2 on the end of the main (which is equal to 4 # 1's)but I still have some whistling on the last 2 risers on the main and I need someone to give me a formula to use on venting the main instead of the trial and error I'm using