Max, glad to see you found this thread.
We’re not measuring intake manifold vacuum. We are comparing gasifier vacuum at the engine to the vacuum behind the air valve. If these vacuums are the same, you have equal pressure of woodgas and air, thus a 50/50 mixture.
Here’s my best explanation:
The valve position will change depending on the measured vacuum of the gasifier. It will automatically open the valve if the gasifier is producing more pressure like at a stop sign. Then it will close again when you whack the throttle and that gas pressure goes away. Under hard acceleration the gasifier presents more restriction, so the valve should close slightly. Those of you with woodgas trucks will note that closing the air valves on a hard pull will often give you a bit more power, this is the same thing.