Rather than measuring the size of a cup, I would go by the amount of coffee, as in the weight of the beans, used to brew it.

I feel this is more precise than the ml cup measuremnts, but if you wanted to be really precise, you'd have to specify the type of beans used (the caffeine content varies widely) and even the brewing method https://oldchicagocoffee.com/coffee-bean-caffeine-content-by....

And - there is an influence - even in the region the beans are grown. In the link I provided they even go so far as to differentiate as to genetics of the beans.