charging and frightful presence range

[StDrake]

So my search-fu failed me.
If a creature has frightful presence activated during a charge, when does the effect kick in and where does it spread?

assume 20ft movement (common humanoid turned into a nightmare creature in heavy armor)

a)at the start of the charge and 30ft from the starting point
b)at the end of the charge move so 30ft from the landing point
c)all the way through the entire thing, effectively 30ft around both starting and landing point + the 60ft wide charge distance long rectangle inbetween?
d)?

my logic suggests point c, but both me and my GM feel something wrong with a 30ft radius turning into a 60x100 rounded rectangle

[James Risner]

Frightful Presence is a Ex and can be activated as a free action when doing an attack or charge.

It doesn't specify explicitly, so you will see table variance.

Your best bet is before or after the attack, before charging, or before or after the attack from the charge.