Wildturkey, there are some situations that limit your qualification range to less than 2X the test coupon. I don't have them all memorized, but GMAW short arc limits the thickness to 1.1X test coupon (for coupons less than 1/2").
Arcblue, it would be a good idea to review Sec. IX before undertaking performance testing. There are a number of essential variables that may cause you some difficulty if you miss them (diameter range, position, etc.)
Charles Hall
With certain exclusions, as pointed out, you can qualify for the "Maximum to be welded" which is normally interpreted as unlimited thickness if the weld is 1/2" and over, and has a minimum of 3 layers of weld metal. See QW-404.30.