[s-cars] BB Upgrade - rotor question

[Tony Curran]

Hi folks, I'm accumulating the parts for a spring upgrade to Big Blacks.
Just received a used pair of 993 calipers and was doing a bit of cleaning
when it occurred to me the space for the rotor didn't quite appear to be
30mm wide - measures just a tad over 28mm.
 
So, when the dimension of the A8 rotor 4D0 615 301 J is spec'd as 323x30,
just exactly what is the 30?
 
I'm planning on using plain A8 rotors (no slots or holes & $95) any thoughts
on pros/cons? (I don't plan to track car).
 
Thanks
 
Tony
96 S6