Have you looked over on Large Format Info yet? (www.largeformatphotography.info/forum) The die-hard ULF lens experts over there will be able to tell you for sure. IIRC the stated 12x20 coverage is correct, but how much beyond that I don't remember - it may cover with 1-2" to spare, so you're not going to be shifting it horizontally or vertically very much on 12x20.