It's based off of empirical data, so yeah, it has some validity. I think the LSAC calculator is a pretty useful tool that should be used simply to give you a BALLPARK starting point of where to start when applying. If the calculator says you have a 0-5% chance of getting into that school, that should tell you something. However, it's a calculator, and it is based only off of GPA and LSAT. If you have incredible soft factors (I'm talking about working 10 years in an NGO), then it's different for you. Also, it's different if there's a great disparity between your GPA and LSAT (you have a super-high GPA but a low LSAT, or vice versa).