Aren't we all stumbling around in the gloom?
I don't have or use a vacuum gauge, but it monitors a single variable. It is useful for comparisons and diagnostics, but doesn't directly give what you want. Even a new car with an instantaneous read-out is averaging fuel consumed (or so I understand), measuring the flow of small volumes of liquid is non-trivial. The solution I saw used small pins or columns in the flow and then measured the vortex street that results.
Look into
Brake Mean Effective Pressure.