ok what raymond has said is great. hope it helps.
but I'm more interested in is why you would want to tune the guitar instead of transposing. As it is, the guitar is not tuned to any particular "key" per se. I suspect you are doing it because
1) you are unsure of how to transpose
2) you want to be able to sing all songs? i.e in a certain key.
First of all you must know that even though you have tuned the guitar down, the resulting key may still be too high for you and you still will have to transpose. I'm only guessing here, maybe if you would like to tell us why you want to change the tunings instead of transposing we could help you easier? I must say though it looks and sounds difficult, transposing is not that hard. I transpose songs down all the time when I'm playing them for fun alone cos I have a low voice and I can't reach the higher notes