One of the answers was edited to “portion”, which is the same idea, I think either portion or proportion is elegant and simple. If anyone has been using proportion then I’d stick with that, though portion is nice and succinct.
[1] https://www.dictionary.com/browse/proportion
[2]https://www.merriam-webster.com/dictionary/proportion (3rd. entry)
A portion of fries can be 80 individuals, the proportion of all fries in the fryer might be 0.1; proportions are often expressed in percentages.
So "the proportion of the population using my tool is 1%" means 1/100, ie 0.01 of a population normalised to unity.
Pro-portion is like pro-rated, it's taken as in comparison to the whole (normalised to the whole, if you like).
"What proportion did you contribute" == what share of the whole did you contribute.
[en-gb native]