I seem to have a better understanding now (RTFM!) and found this is stable 

```rust
    let mut pwrcfg = pwr.ldo().vos1().freeze();

    let rcc = dp.RCC.constrain();
    let ccdr = rcc.sys_ck(100.MHz()).freeze(pwrcfg, &dp.SYSCFG);
```

VOS1 is for normal operation up to 400Mhz, but 400Mhz is unstable for me.  100Mhz is fine.  I also found this as a reference for vos0 https://github.com/stm32-rs/stm32h7xx-hal/blob/master/examples/vos0.rs and https://github.com/stm32-rs/stm32h7xx-hal/blob/master/examples/voltage_scaling.rs