I have a bike that fits to exactly and I've taken two measurements to see if a used bike would fit. One is center of BB to top of seat along the seat tube. The other is the horizontal center of seatpost to center of handlebars. I figure I could adjust reach with a different stem.

I ask because I'm shopping for a used and that best bike might be smaller or larger than my current bike. Am I oversimplifying? I know there are other geometries that affect positioning but is this enogh to get me in the ballpark. I'm fairly certain I could replicate handlebar drop and BB setback I could adjust with a setback/non setback seatpost.

And a related question on stem length, how short is too short, how long is too long? I was thinking 90-120 as good and 80 or 130 as borderline acceptable.

Decent plan for evaluating used bike fit with no shop to assist you?

Top tube length will set you in the ballpark. Seat tube length will be a not so close 2nd. The correct stem length is the one you are comfortable with.

Not seat tube length per bike spec. Seat tube length per BB to the top of MY existing SEAT, for me 76cm. As long as I get the new bike's seat post up that high or down that low am I good?

And also top tube length not by bike spec but by actuals. For example, my current bike has a 52cm top tube and a 110mm stem. A total tube tube length of 63cm. Can I get a 49cm top tube with a 140 stem or a 55cm top tube with an 80 stem? It seems like we may fit a lot more than just 1 or 2 sizes if adjust stem length.

It's more complicated than that, but most people can make a bike a size up or down fit them if they need to. But everyone is built differently. I have to concern myself with seat tube angle more than anything, but I'm just an odd duck.

Still, I've noticed that going up a size with a shorter stem or going down a size with a longer stem tends to make the bike feel different and handle differently.