40% is still a large number even for post menopausal women.
To me, 1/100 would be rare. I've seen many young women in her 20s have more advanced thinning than I do, including two good friends of mine. They never did anything about it. And surprisingly these women are largely oriental/Indian women. I've also seen quite a few white young women with very diffuse or advanced hair loss. They manage to cover it with a combover but were never afraid to show their thinning.
So from my day to day observation over the last 5 years in various countries (I love backpacking so I travel a lot), its crazy how common hair loss is in young women nowadays. It's certainly not common. Most cases just go unreported.
Also, how would you even collect your stats as from a lot of women's experience I've read, doctors' first response to a female patient's hair loss issue usually is "you're fine".
Generally speaking, if something occurs less than half the time (40%), it is considered "rare" or at the very least, "uncommon".
The numbers are approximate numbers based on patient observations made by dermatologists and other doctors. It's an easy experiment to do and easy to collect a sufficient sample size in a short amount of time.
All you'd have to do is look at how many patients have the onset of or advanced Androgenetic Alopecia, sort them by sex, count them and then compare them against others' data.
Next time you're in a heavily-populated area, use the notepad on your phone and tally how many people you see with hairloss and sort them by sex; almost every time, you should see that the number of men significantly outnumber the women.