The key features of the International System are decimalization, a system of prefixes, and a standard defined in terms of an invariable physical measure.
Then I pick a key metric, and I ask myself if the metric is specific, easy to objectively measure, and representative of true progress towards my goal.
A useful metric therefore is the FEV1 to FVC ratio, which since the FEV1 goes down even more than FVC, causes the FEV1 to FVC ratio to go down as well.
I find that a good metric has the following traits: it's very specific, it's easy to objectively measure, and a change in the metric indicates progress towards or away from my goal.
By the late Middle Ages, the Hindu-Arabic decimal system mostly replaced Roman numerals and fractions in Europe, but efforts by scholars like John Wilkins to promote standard decimal-based measures were less successful.
He was a stickler for standardization. He wanted the wheels on carts to be the same distance apart so the ruts in the road would be uniform. He also standardized all weights and measures.