i m aware about the GN calculation...for optimum result...
GN = Aperture x Distance
and the distance mean 'from camera to subject'
but...are this apply to the distance from the flash to subject...
(situation - subject in 12 o'clock)
(situation - flash in 6 o'clock / 45deg toward subject)
(situation - flash in 9 o'clock)
(situation - reflector in 2 o'clock)
hope this will help..
thank in advance