Link in the fourth paragraph above (http://www.lafavre.us/tuning-marimba.htm) The essential bit of info is a picture of a Marimba bar not quite half way down in the article, showing the approximate areas one would grind into to tune the tonic, 2d (bending), and 3d (flexure) modes of the bar.
Of course a Glock bar should be kept as nearly a pure rectangular bar as possible, so the deliberate "arch" used on a Marimba bar greatly exagerates how much you should cut in.