Like has been said - leave a little. Remember, if after the end of it you're even a teeny tiny bit shorter than 18 it's an illegal gun if the cops ever wanted to push the issue. That's why most gunsmiths actually don't go much shorter than 18.5, just to leave a little incase something needs to be done later. The difference that extra half inch is won't make any real noticable difference when you carry it, but it just makes sure you've got a little extra room so there's no 'doubt'.