Ditto. Proper bike fit is a journey, not a destination. I'm constantly changing fit on my fleet of 6 currently-running bikes.
What I do first is get it close, visually. I can tell by looking at a bike whether it fits me or not. Once over that hurdle, I get on it, and go into the drops, and look straight down. I wanna see my handle bar drops blocking out my front axle, or very close to that.
Then I just ride it and see what starts hurting. Here's a partial list of how to fix some common problems.
Knee sore in front = saddle too low.
Hamstrings sore = saddle too high.
Back feels compressed / rounded / can't breathe properly = longer stem
Lower back sore = stem is too low, raise it a cm or so
I like my seat perfectly level, and drops parallel to the ground. It's not that hard to dial in a bike that's within my range (54-58 cm) just takes a few days and some patience. The final test is a day in the mountains. If I can spend a half day or more on the bike, and nothing hurts afterwards (and I don't crash) that's when I stop tweaking on the poor thing.
