Call of Cthulhu Character sheet question with Sanity

Okay I've been having this problem for a while. The sheet has "Current, Start, and 99" and the book has "Characterstic, Maximum, and Current." Maximum = the 99 as it lowers with increased with Cthulhu Mythos, and the Start is your Pow. But what does Current equal then? the 99? then they'd never lose sanity. their Pow? then if they have low willpower their doomed from the start. TL;DR, What does Current Sanity equal. Pow or 99. Or some other number

Current SAN  starts at same as your POW.  Average SAN is around 55, average SAN loss is 1 for a successful SAN roll and 1D6 or 1D8 for a failed roll. SAN is regained as rewards for finishing a scenario and through psychoanalysis.

Ahh okay, thank you for the clarifications.