You need to do the calculations for calories.
Ex. assume 100 grams of this
55 grams protein * 3.5 calories per gram
0.5 grams fat * 8.5 calories per gram
100-55-0.5-2.0-35 = carbohydrate ?
If yes, then 7.5% carbohydrate by weight
7.5 grams * 3.5 calories
Next, you sum the total calories.
Divide the calories from carbohydrates by the total and multiply by 100 to get the percent of calories from carbohydrates.