Davin examined every issue of every science fiction and fantasy magazine published in the US between 1926 and 1965, looking for women writers. First he limited himself to magazines that were devoted to either science fiction or fantasy (and admits there were women who published science fiction and fantasy in general fiction magazines that aren’t included in his count). Second, he only counted authors who could be verified as female. Any author with a gender neutral byline or who used initials who couldn’t be verified as female weren’t included.
Davin goes into detail in his chapters, describing and/or quoting some of the authors at length. He also conveniently provides tables and summary figures in Appendices. From 1926-1949, 65 female authors published 288 stories in 20 magazines (i.e., all of the genre magazines of the period), From 1950-1960 another 138 female authors joined the field, for a total of 203 women writers who together published a total of 922 stories.
These figures don’t include women who published in Weird Tales. In The Unique Magazine, 127 known women writers published 365 short stories and serials, or 13.45% of the fiction. These figures do not include female poetry authors (63, or 40% of the poets), nor do they include authors of indeterminate gender.
Now that is how you go about testing a claim — set up the most restrictive accurate definition you can manage, exclude neutral or unverifiable possibilities, and separate out borderline cases especially when they’re very strong examples.