Gotta be mathing it based on absentees left to count and an estimate of Biden/Trump share.
If we know what the Biden/Trump share has been thusfar county by county, then the math should be fairly straightforward. I’ve found overall PA absentee results (78% for Biden), but I tried looking at one specific county (York, for example) and couldn’t find their absentee vs. election day count split out. So I’m not sure how people doing the math account for potential uneven distribution of absentees remaining vs. already counted.
Edit: So, this is assuming the AP numbers are correct, and I’m not accounting for rounding which could make a difference. But if Perdue wins 42% of the remaining vote, that would get him a final percentage on my spreadsheet of exactly 49.9997% LOL.
So ~42% of remainder is the breakeven for Perdue to avoid the runoff. I think Ossoff makes it.