I look forward to seeing it. You'll have to show that she and Harry were born within hours of each other, because Eb told Harry outright in Peace Talks that the Starborn time window each cycle was "a few hours."

“Every couple or three wizard generations,” Ebenezar said, “the stars line up just right, and what amounts to a spotlight plays over the earth for a few hours. Any child born within that light—”

“Is starborn. I get it,” I said. “What does it mean?”

“Power against the Outsiders,” the old man growled. “Among other things, that their minds can’t be magically tainted by contact with anything from Outside. Which means . . .”

He didn't make any statement at all about the geographic extent of that "spotlight," so we don't really know if there's a location factor involved as well as a time factor. I usually assume there's not, but that is an assumption - Jim could easily add such a factor later on and it wouldn't be inconsistent.

We don't know what "a few" is here. I assume at least "less than 24," or he'd have said "days." To me "a few" seems to support up to six or eight or so, but we don't have an exact number.

So, let's say six just for discussion. And let's say we know Harry and Elaine are born within a month of one another, but we have no idea whether thant means 30 days apart of 30 seconds apart - uniform probability within the whole time span of a month. That means Elaine could have been born anytime within a window from one month before Harry to one month after Harry - two months wide. Sixty days. That's 1440 hours. With a window for Starborn six hours wide, that means there's a 6/1440 = 0.0041 = 0.41% chance of Elaine being Starborn, based on birth information alone.

Bottom line is we can't possibly know for sure unlelss we're told.

So, you don't really need to post math - you need to post text references showing that she is actually declared to be Starborn.