I must admit that I tend to guestimate, too, based on occasional bench mark meals at home, when I can weigh food. Fortunately, most of the produce we get from our garden (eggs from ducks, various veg and berries) is vey low carbohydrate. We make our own bread, so that's pretty constant - the only variable being how thick I cut my slices.
I will be volunteering on a week long trail run in Wales in September - food (breakfast, packed lunches for the hill / midday check point, snacks & soup on arrival at overnight camp, evening meal, drinks whenever camp kitchen is open) will be provided for all racers & marshals, all vegetarian, and portion size will be basically what you want to eat. As participants provide their own plates / bowls / mugs, I might check what my normal portions look like in the crockery I'm taking, but otherwise, will take a chance. There's no way I could bother with carrying a set of scales, and that would cause interuptions to meal queues.
I reckon that using techniques to improve guestimates of carbohydrate content of food is the key to a happy life - there are so many other variables, including exercise / stress / sleep hours / meal times / weather etc, that there's no point being ultra accurate in one factor, when it's impossible to have the same level of accuracy for other inputs.