The penult, if accented and long, takes the circumflex when the ultima is short by nature (νῆσος, ταῦτα). In all other cases it has the acute (φόβος, λελυκότος, τούτου).
a. Apparent exceptions are ὥστε, οὔτις, ἥδε (properly ἧδε). See cross186.
b. A final syllable containing a vowel short by nature followed by ξ or ψ does not permit the acute to stand on the antepenult (οἰκοφύλαξ); but the circumflex may stand on the penult (κῆρυξ).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].