Suzi, Here's a way to cheat without so much math. Set the crop tool's aspect ratio to use photo ratio. Then you can crop using the 2:3 ratio that your camera outputs. It won't let you stray from that ratio. Just have to decide what parts to include if need to crop in further.
Well, Lightroom has an "Original" setting in it, but if you are cropping from CS3 there are only presets and none of them are equivalent to the Use Original thing. You have to know what you are trying to get to in terms of the aspect ratio.
Besides, if you don't know and are trying to make it fit something in particular, you have to be specific or you will not get what you are after.