I'll take a shot at it.  My best guess is that you establish your own tolerance and follow the test criteria in 7.6  When the base material is vertical, greater care is required to ensure that the stud is perpendicular to the work piece, so your tolerance would need to be tighter than the flat tolerance.  Since there's a tendency for the weld metal to run to the underside of the stud base and run through the vent holes in the ferrule, this creates an uneven weld and failure to obtain a full 360 degree flash, particularly at the top.  The effect is more noticeable as stud diameter increases and generally speaking it is not recommended that studs of over ½” in diameter be welded to vertical plates for this reason. The essential requirement to obtain satisfactory weld formation is to use the shortest weld time possible with increased welding current.  On small diameter studs, vertical welding isn't much of a problem because the weld time is short and there's not much weld metal produced.  In case you're not aware, regular ferrules do not work well for this application.  Special ferrules designed for welding to a vertical surface need to be used.  These ferrules have blocked vents at the bottom of the ferrule that help keep the weld metal in place, which in turn helps in maintaining a full 360 degree flash.