Well, first off, there's a wealth of posts/discussion/information/references on this subject in the Fitting Your Bike sub forum. As to the second part/question....everyone's different as to how much trial and fitting they need. Some highly experienced can order out custom with confidence. New folk, it should take a lot more trial and dependence on a good LBS with good people to get the right fit. I'm a bit in between...not good nor confident enough buy without trying, but have a good LBS and it took me less than a minute on the bike I bought to know it would work out just fine with a few fitting tweaks.