I always put my fingers in the "gap" behind his elbow and slide down. The girth puts the most pressure on the sternum which is right in the middle. So just putting your fingers on the side is not really a good way to judge because some girths might feel looser/tighter at that part depending on the shape of the horse. The sternum is really the important part for judging tightness.
Just thought I'd mention that since no one else brought it up.