To be absolutely sure of the measurements in making fuel, I prefer to use a scientific graduated glass cylinder. And measure each component individually and mix them in a different vessel. That way I know exactly how much of each has been added.

This is the only way I know to get consistent runs, because I know the fuel is mixed consistently.

Mixing in plastic bottles relies on them being marked accurately and not expanding or contracting with the weather. Also as previously mentioned, intermolecular dispersion can affect your initial measurements if you combine at measurement stage.

Regards

G