How do we know how much Creatine supplement to take to keep our diluted urine in the “normal” range for creatinine?
Youre not going to know - not without knowing what your "baseline" creatinine level was prior to diluting. And you will not know what your baseline level is without testing your urine with a validity test kit.
(Do a Google search for "drug adulteration test kits"...these kits will also check the S/G)

Each persons baseline creatinine level is different. Therefore, what particular dose was sufficient for one person is not necessarily enough for another person.
In order for the urine sample to be not ruled "too-diluted", the creatinine level must be over 20 mg/dL. Creatinine levels between 2-20 mg/dL are ruled too-diluted. If the urine sample is between 2-5 mg/dL, the retest will be done under direct observation. A urine sample with a creatinine level between 0-2 mg/dL is ruled "not consistent with human urine" (aka "substituted specimen").

There are no upper limits established for creatinine. As long as the creatinine level is over 20 mg, its satisfactory. However, this doesnt mean you should go overboard with creatine supps. Too much creatine can result in long unwanted bathroom sessions, sitting on the toilet seat.