I have a 3240 with a 54" deck, my gauge goes almost all the way to 6 when fully lifted, and the wheels are definately off the ground. The gauge not being exact wouldn't be a big deal to me, I would just find where I like the grass cut and remember what the setting is, whether it's 2, 3, 5, whatever. It doesn't really need to translate to inches.
On the other hand, you did just pay a lot of money for it, and if it is important to you, you could take it back and ask that it be fixed. That's not at all unreasonable. I'd probably wiegh it out though, how important is it, how long do I want to be without my tractor, how much time do I want to spend talking to the dealer, driving back and forth, etc. Either way, good luck, and enjoy your new toy... er I mean tool /forums/images/graemlins/grin.gif