Looking at an HC table online they have about 55grhc/100gr, that is, the ration is 18gr.They have IG 40. A ration of whole wheat bread is 25gr. The slices that you take the weights and know the equivalent to the flakes.
Check in the box information how many carbohydrates have in 100 grams of product.Then divide 1000 for the amount of HC in 100 grams and you have how much a ration weighs.