The simple answer to this is, the more voltage the more effective, but also the bigger the issues with skin irritations.
On the Drivers we set the voltage to 10V. It can be turned up to 15V.
People who want to use the device at higher voltage usually have to resort to gel electrodes or handheld metal electrodes to avoid skin issues.
Or you can put the wrist straps on your ankles as a second location to minimize any irritation.