You're on the right track. If you have the same weight, the yardage will be pretty similar as the original, but if it's just a bit off it won't matter (I just bought skeins that were 100g/242 yards to sub for another that was 100g/260 yards). Just make sure you order the same total yardage.
I do, yes, usually order an extra ball just in case. I've found myself a few yards short before! I think whether there's a buffer probably depends on who's writing the pattern. Better safe than (trying to match dye lots later) sorry!
RE: a book or web site, I don't know of one...just trial and error speaking here, but someone else may have more ideas!
Also, Knitpicks is great, and even if you were to get your yarn and find it's all wrong, they'll let you return it. Go for it!