for zerks and pures when you're at like a combat where people use whips etc i'd assume 52 prayer so u could make some bank and its easier not to get smited aswell.
I'm currently 31 prayer on my pure to keep my combat low. Downsides of this are that I can't get myself a fire cape, some slayer tasks are hard because I can't use prayer and ofcourse questing. But my stats are somewhat better as others on my cmb with higher pray etc. Only quest left on my pure is DT, which I'm waiting for my friend to complete it so he can block