Hear what your saying - covering as many possibilities.
Minimum water 15ltr on top half of CSR.
D Doc - 6ltr down from previous trip from 18ltr.
Suppose we all calculate risk against the variables. Temp at the time -- how hard the rider is working and sweating.
Did a ride south of the Kalahari desert in Namibia a few years ago in late summer (dumb thing to do). I remember drinking 9ltr that day and still felt thirsty that night.
My personal take is to drink enough water so you feel like a piss before you hit the sack.