humans are not different sizes, they have the same ratios
Having grown up with mother & sisters, dated women & been married, I can tell you women have different proportions. I have been inflicted with long discussions on which brands of clothing fit which women. Avoid all women once you hear the words "shopping" and "business casual"
the span of 9 inches (finger to thumb) and cubit of 18 inches (finger to elbow)
Wrong. My little finger to thumb is 8 inches. Thumb to index finger is 4 inches. Tip of middle finger to outside of elbow is 17 inches. Tip of middle finger to inside of elbow is 14 inches. My foot is not a foot long.
All human measurements are arbitrary so all your ratios are arbitrary.