Apart from setting the range properly, there aren't very many settings.

What have you tried? Could you elaborate on what you mean by "twitchy?"

Hi mzed,

From what I've seen it's virtually impossible to get a stable result. Even the help patchers for fzero don't appear to work that well, even if I use a sinwave at 440hz, so I'm not surprised that a singing voice isn't giving me any meaningful data.

Try upping the period to 512. Sine waves are hard, because they have no harmonics. This should be solid: