I used to have a notebook in which I wrote down the percentages for various foods. I use net carbs, so minus the fibre.
I used raw weights, so I could just put a pan on the scales, zero it and then add in the food up to the amount I worked out - if something is 10 percent carbs then every 10 gm of the food is a gm of carb, 5 gm of carb, 50 gm of food - I did keep a sliderule in the kitchen so as to work out the more difficult ones.