Hi Bungle, I've just looked up flax seed on Fitday. It gives 53 carbs for a cup (about 200g I think) and 43 carbs is fibre, so since you subtract the fibre, 10 carbs effectively for 200g, so 5 for 100g, which is what you thought.
Hope that's useful. Do you know about Fitday incidentally? Find it at Fitday.com, its free and very good for tracking food intake etc. Ignore this if this is egg-sucking advice!
Take care, Jay