You can find/build anything and everything off of a centerline string. Your garage or shop build should ALWAYS start with one. A bit of a pain to set up for the novice, but once you learn it, it is very easy, but very time consuming. I wear out tape measures when putting these things together! And Mike is right when he says 3/8 is close on a stock framed car...you have already found 1/8 of the 3/8, and I am betting if you did a centerline string you would find some more of it, or find where a small mistake was made. Generally we always strive to get it dead nuts, and sometimes you get it, but in reality a 1/8 plus/minus tolerance is the general rule.