I would say that any reading below 50 requires immediate carbs to bring up. If below 40, use karo or honey. In the 40s, use wet food, the % carbs depends on how your cat reacts to food.
I would also recommend testing for ketones regularly with a diabetic cat, even if the numbers are in the normal range (50-120 on a human meter). How often depends on how prone to ketones your cat is and how high the numbers.
Just my $0.02 worth.
Liz