You seem to have the general idea. I agree that you should be careful about buying into a lot of the BS that surrounds bike fitting. I've had good experience with two fitters here, both of whom fitted the old fashioned way--watching me pedal in the trainer, with no "systems" or body scans.

From that experience, I learned that even though two of my bikes are different "sizes", (one a 56 and the other a 58) they both have the same top tube measurement, within a quarter-inch. And for me, it's the top tube measurement, not the seat tube (which is usually the one used to determine frame size) that makes the difference.

Recently, I purchased another bike, used. I bought it from the LBS who sold me my first bike, which was a size too small. He told me this bike was a size too big. I measured the top tube. It was spot on. A test ride confirmed it--fit like a glove, so I bought it. On my first 20-mile ride on it, it felt like old home week.

There are two morals here: 1) That this particular dealer tends to undersize, and 2) I should always carry a tape measure when I buy a bike. A 22" to 22½" top tube (or in the case of sloping, the "virtual" top tube) is what fits me.

Do your own measurements of bikes you have that fit, and those that do not. Bring that information with you, and start from there.

EDIT: The other measurement that's important for me is crank length. 170mm feels like I'm taking baby steps, or the pedaling equivalent of pulling punches. 175mm fits just fine--it feels like I'm using my whole leg. So any bike with 175mm cranks and a 22-22½" top tube is going to be right for me, with only minor saddle and bar tweaking.