Most references I've seen don't bother with a formula but simply suggest a target figure of at least 2 litres per day, although some sources suggest a lower figure for women than men, with women only having to aim at 1.6 litres per day.
The current 'official' advice (cited in both Government and NHS publications) is that from the British Nutrition Foundation, which says that adults and teenagers need 6 to 8 glasses of liquid per day:
https://www.nutrition.org.uk/healthyliving/hydration/adults-teens.html
In years gone by I almost certainly didn't achieve those amounts but I spent most of 2019 with a catheter inserted into the most delicate part of the male anatomy and various doctors told me that, to prevent any possibility of blockages, I should be aiming for at least 3 litres of fluid intake per day, and that 4 litres per day might be even better. Even though I had the catheter removed a year ago, I continue to keep track of how much fluid I drink each day, which is usually between 2.5 and 3 litres.
Every loo in our regional hospital, together with those in my GP's surgery and in local clinics, seems to be plastered with urine colour charts, such as this one:
https://pbs.twimg.com/media/DgoIK3LXcAASUXQ?format=jpg&name=small
So the idea that we should all keep an eye on the colour of our pee, in order to monitor our hydration levels, seems to be well established and widely accepted.