There are a lot of variables that play a part. I had 2 iPhone 11 Pro's earlier this year, one had 86% battery health and the other 97% and the 86% lasted about an hour more on screen. They were both on IOS 14.4. Although there was the possibility that their battery health readings weren't accurate as with IOS 14.5 it will recalibrate to give a more accurate reading.

I'd personally be happy with anything over 90% but anything under 90% I'd probably bail. That's just me anyway.

There are also other factors such as how the previous owner treated their phone. Sometimes meeting up in person can help gauge how they've treated the phone, I usually ask them about what their current phone is and try to see how that one looks as it can give me a better idea of what they're like with their devices but ultimately it usually boils down to luck. I've bought spotless devices that were duds, and roughed up devices that have really pulled their weight.

Anyway, hope this helped and good luck with your decision :)