I don't think it's possible with any method to get closer than about 10mm to the "correct" length for a given rider without in-person fitting followed by a tryout of the result. Whatever method you use, I would advise trying the result out with an adjustable stem that will allow you to vary things enough to find the just right length before going for the permanent one. If that is not available it is fairly easy to mount and remove a stem without any cosmetic damage, and exchange it for another until you find the right fit.

I agree that the handlebars can affect which stem will work best. I changed my stock hb out to one that comes forward quite a bit more and that also has a significant drop. That results in a shorter stem than would want with bars that have a shorter throw and drop.