I keep mine tuned 4 frets down - I discovered by accident one day that that's where it sounds best, and capo up as needed. Nick usually assumes that the 12s will be at least two frets down.

That means that for me, C is A, D is Bb, and G is E (I think....)

Make sure you note the string gauges that Nick recommends. I always make up my sets custom, with nickel wound 17s for the string that, were the instrument tuned to regular guitar pitch, be the B.

Dan