Bike sizing...can it be that difficult?

I think my bike size is a 58cm but that's not really logical.  There's no empirical data beyond questions about "how tall are you?" and "start pedaling and let me see how you look."  It goes against my better analytical judgement as I keep hope that there's a magic combination of height, inseam, flexibility, etc. that will guarantee me the right fit.  I'm on a 60cm bike now but it turns out that it's maybe a 58cm frame with an obscenely long stem and non-compact handlebars.  So then a 58cm frame feels about right but after a local bike store has put me on 58cm frames twice they're now thinking I might be a 56cm and just need a longer than average stem.

Can't someone invent a nice app for my iPhone that can take a picture of me and magically "fit" me to bikes from Cannondale, Pinarello, Specialized, etc.?  It sure would make life easier...