proporcio

No affix here, despite appearances. The -ci- in proporcio is not the -ec- of abstract qualities and not any other suffix in the language. It is just the tail of the Latin word, arriving whole. The same applies to porcio, a portion of food, which is a separate borrowed root and not a piece of this one. Compare procento at rank 1,995, another Latin pro- phrase that came in as a single lump: pro cento, by the hundred.

Two senses, both live. The mathematical one is a ratio between quantities: la proporcio inter A kaj B, and the four-term version A rilatas al B kiel C al D. The everyday one is a share of a whole, which is where English speakers reach for percentage and Esperanto is happy with either proporcio de or procento. The third, aesthetic sense is about a thing being well shaped: a building, a face, a sentence. The adjective proporcia and the compound senproporcia (out of all proportion) both do useful work.

Where it sits among the measuring words you have.Kvanto at 1,438 is how much, nombro at 576 is how many, parto at 286 is a piece of a whole, and proporcio is the relationship between two of those. Egala at 588 names the special case where the proportion is one to one. Together they are most of the arithmetic vocabulary in matematiko, and none of them needed a new root beyond this one.