I did a quick search of the forum but didn't find much on this brand other than very brief mentions. It's a fairly new-ish brand so not on the 2017 Catinfo food chart. The Rustic Blends are under 10% carbs, at least from doing a quick calculation using the Excel file download on the famous Janet and Binky's web site and the as fed numbers posted on each Open Farms product listing. It's another food option for diabetics The brand has freeze dried treats, too. https://openfarmpet.com/en-us/collections/wet-cat-food/
No reason for you to do any calculations. If you go to the Nutritional Profile the % calories from protein, fat and carbs is listed. For the chicken it is 5.1% calories from carbs.