The fascinating thing about water intake is that there is no scientific consensus regarding the optimal point or how to determine it, or how it varies between individuals. It is one of those items that you think would have a good answer, given where we are in the development of the medical life sciences, but apparently not.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908954/

"Little work has been done to measure total fluid intake systematically and there is no understanding of measurement error and best methods of understanding fluid intake. The most definitive US and European documents on total water requirements as based on these extant intake data. We feel that absence of validation of methods for water consumption intake levels and patterns represents a major gap. Little exploration of even varying methods of probing to collect better water recall data have been conducted.

"Of course, the other half of the issue is the need for understanding total hydration status. We have no acceptable biomarkers of hydration status at the population level. Controversy exists about current knowledge of hydration status among older Americans. This represents a topic understudied at the population level though certainly scholars are focused on attempting to create biomarkers for measurement of hydration status."