Some people think that because a cat can concentrate its urine, they must. Cat’s blood will have a SG of a little less than 1.015, so as long as a cat’s urine concentration is greater than 1.015 you are good to go. Concentrating urine above 1.035 is a survival tool for desert living, not for healthy living.
The preferred specific gravity has a range between 1.020 - 1.025. For additional information read Hydration: Dry vs Wet.