use a black marker pen to experiment with a pattern, stand back and have a look at the pattern, no good ?... wipe it off and have another try
when you go to drill the holes use a centre punch mark to guide the drill, even with a pillar drill the case can still move when the drill bit hits it, especially with smaller drill bits they can ' bend' as the drill hits metal, the centre punch will give the tip of the drill something to hold onto for that vital first second or two
i know all that sounds obvious but ask me how i know.....