Dr Mike also uses 8% as ash although some are higher or lower. That still can’t account for the real fat content or protein content in a food as they are minumums.
And looking for a food solely based on carb content for diabetes can be misleading or detrimental for some. There are some situations where just the lowest carbs are just not healthy for a dog such as a high fat diet or one with a fat-to-protein ratio above 100%. An active/working dog can handle that much better than a sedentary dog. So feeding a high fat but “low carb” food to the sedentary house dog with diabetes can end up with a dog that has both diabetes and pancreatitis.
There’s never a one food fits all or look for one characteristic of a food that will work for all dogs with diabetes. And unfortunately if you don’t make your own food, you will never know the amount your feeding of fat from a commercial food.
