Back in the days of quill stems, I had a Look Ergostem. Anybody remember those? Double articulating stem that allowed a range of positions from extended gorilla to bolt upright. There wasn't anything like it.
It had a fair amount of flex, but it came in super handy when setting up a bike, almost like your own fit machine for bar position. I'd dial in fit on the Ergostem, then find a rigid stem with the same rise and extension or as close as possible.
These days, I use a Ritchey adjustable threadless stem the same way. Dial in the fit on the adjustable, then transfer that to a rigid stem, or as close as I can get with what's available.
This avoids a lot of the guesswork involved. Also, if you can't get a good fit even with an adjustable stem, chances are that's not the bike for you.