Hi,
A simple thing I do is constantly monitor my urine. The colour should be between clear and light straw colour which indicates your kidneys are getting enough fluid to do their job. All be it you haven't had something like beetroot that can change the colour to red!
I get through about 3 liters of fluid per day plus what is in the food I eat. This has kept me in the GFR 40 range for 12 years now. If you stick rigidly to a kidney friendly diet you can push the GFR up, but it is a struggle to be so restrictive in diet. Eventually I had to relent to a more sustainable diet and not be so regimented. I'm dealing with 4 major health issues which each have their own dietry requirments, so it becomes a balancing act as what is good for one can be bad for another.
One major revalation I learnt is that CKD is not D for death it is merely D for disease. Once you get past the shock it isn't so bad after all, there are things you can do to help and still enjoy life.
Cheers