If a shower is directly connected to a 2" vertical vent that goes through the roof, (only fixture using this vent stack) how do I calculate the branch arm length? Can I divide the branch arm diameter by the downward slope? Say 2" diameter divided by 1/8" downward slope = 16' of branch arm? I actually need 10'. Also hypothetically, can a 3" diam branch arm be used for a shower? thanks for enlightening me!