That's what the dietician is for..

Mine suggests 3 meals a day of 45-60 grams of carbs and 2 snacks of about 15g. To figure out the amount of carbs from a label you take the total carb number and subtract the dietary fiber number. For example, my popcorn has a Total Carb amount of 23g with 9g of dietary fiber so the total carbs for a serving of this popcorn is 14g which is right on target for a snack for me.