I always go by the pack, the atkins book and website I find to be very american and mostly measured in cups and I haven't a clue what that means really, and everything is different.
Tomatoes for instance on the website it says 1 tomato (red, raw) is 4.3 carbs which is a little difficult as they are all different sizes, in fact they are 3.1 per 100g (as per my pack from tesco) and the tomatoes I have are only 70g so therefore my tomato is just over 2g not 4.3
I have done myself a spreadsheet with carbcounts per 100g for most of the higher carb items I eat and then I just enter how much I am eating to get the total carbs for that item. I don't weigh everything I eat I have to add, I am not totally OCD, LOL
There are some items that don't have labels and then I refer to the book/site or just not eat it.