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There are 20 fl oz. to a pint

Instead of cups you should use half pints.

There are 8 pints to the gallon.

Unless you specify which pint, gallon then you're probably wrong anywhere outside the US. Even then you could have to deal with vintage Canadian equipment with imperial labeling.

US Cups are random in measurement and only sometimes half a pint.

The imperial fluid oz. has one value 28.413 ml

The US fl. oz used to be 29.573 ml. But now can officially be 30 ml in some settings.

Metric is the best system, followed by imperial which at least is still a consistent standard.

Then US customary measures where the written value may or may not have to meet a standard these days.

The US has been using metric for everything important for a long time now like the rest of the world. Except the Mars probe NASA crashed.