What is the value of SAN compared to "one-fifth of the current mental health" to determine unlimited craziness?

The rules of indefinite madness stipulate that it comes into play after the loss of a character …

a fifth or more of the current mental health points during a single day

Since SAN losses can occur at various times of the day, what is the "Current mental health" against which & # 39; a fifth & # 39; is measured? Is this the SAN character at the beginning of the day?