An empty cylinder should measure exactly as much lighter than the air as the air weighs. That point is your zero point. Put in helium and you'll eventually rea

An empty cylinder should measure exactly as much lighter than the air as the air weighs. That point is your zero point. Put in helium and you'll eventually reach and exceed the parity point as you keep filling up (same weight as the air), and the ratio of how much it moves for x volume of additional filled gas tells got the additional weight. Soon enough you'll reach 2x air weight and you'll have tipped the scale exactly.