What's the substitute words for "you" in formal writing?
Humans need 2 liters of water. When the water content of the body is reduced by 2% we will begin to feel thirsty. At 5% your head will begin to hurt.

I've been taught to refrain from using 'you' in formal writing. If a writer has already used "Human", "We" such as the quoted sentences, then what is the proper substitute word for "Your head"? Or is possessive case a exception?

Question

What's the substitute words for "you" in formal writing?
> Humans need 2 liters of water. When the water content of the body is reduced by 2% we will begin to feel thirsty.  At 5% your head will begin to hurt.

I've been taught to refrain from using 'you' in formal writing. If a writer has already used "Human", "We" such as the quoted sentences, then what is the proper substitute word for "Your head"? Or is possessive case a exception?

Answer

There are several work-arounds, but if you’re interested in *formal* writing, the easiest to use would be “one”. For example: “At 5% one’s head will begin to hurt.”

Answer

Adding to Phil's excellent answer in the example you have given it is possible to use "one" earlier "...reduced by 2% one will begin" and then use "your" "At 5% your head..."