If you are making it yourself, in fact if you make ANYTHING yourself you need 2 things - one is a really accurate pair of 'Add & Weigh' scales, a calculator and the other thing is Collins Gem CALORIE Counter book. (Not the CARBOHYDRATE counter, that one's far too vague for useful kitchen use)
Then look at your recipe and work out the carbs for everything that's in it, remebering to mutiply the carb value by the correct number of grams.
Then if it's something that stays in a dish, weigh the empty dish.
That gives you the total carbs for the entire article. When you dish it up, you will know if you've had half or a sixth or whatever, so divide that into the total carbs and voila! - you have the rough carbs per portion, don't you?
You can actually be more precise than that. Say a pudding. Work the carbs out from the recipe as before. When it comes out the oven weigh the whole caboodle, and take off the weight of the empty dish from the whole and that is the weight of the pud itself. You know the total carbs. Work out the carbs per 1g of weight.
Put your individual serving dish on the scale, when it tells you the answer, zero the scale, dish up your portion and the weight it gives is just the weight of your dollop. Multiply that by the carbs per gram and Bingo, that's the exact carbs in your pud.
Sounds terribly complicated but it isn't!