I have some questions about bike dimensions:

1. Is it possible to determine the desired effective top tube of a bike with drops if I know the effective top tube of a flat bar bike (Specialized Sirrus 2012 to be precise) that fits me very well? From the way fit is described a formula should be possible right? Does one exist, and how well does it work?

2. Why doesn't the effective top tube length factor in length of the stem? I mean, if you have keep the same effective tt length and change the stem, everything gets messed up, right?

3. When trying to determine basic fit, apart from effective TT and standover, what are the other important measurements to consider?

Context: The goal is to determine a used bike ad on craigslist is worth a test ride. Can't possibly test ride all, but can look up specs online and then try to do a basic match before setting up an appointment.