Here's a tip for getting that centre mark right, if they are through holes or if the bolt is broken off below the surface. Grab a drill bit the exact diameter of the hole so that it self centres and lightly drill a small indent into the bolt then use small drill bits and work your way up to extractor tool size. If a bolt is snapped off but not seized sometimes using left hand twist drills in a reversible drill will get it to run itself back out as the drill cuts.
