FWIW - What you were told about the rotors being hubcentric and not relying on the bolt pattern to centralize them is 100% correct. The last rotors I redrilled actually had 17/32" hole in them from the factory. That's 1/32" larger than the 1/2" stud. At that rate, using the old rotors as a guide and just lining the center bore up will get you plenty close enough. As long as the center bore fits snug over hub and the studs go though the holes, you're good.