If you go on to many different sites, like
Tire Rack - Your performance experts for tires and wheels for example, they will tell you the proper size.
I know that for the 18x9's, 255 is the proper width. I am planning on getting 18x10's in the rear and running 275 or 285 on them myself. As long as you recalibrate your speedo with a tuner, your speedometer will not be off.
Anything bigger than 255 up front and you run the risk of rubbing.
American muscle actually lists the proper size right on the page of that link you posted btw