From this post, I understand that the flash guide number (GN) is given by the following formula:
Guide Number = Shooting Distance × f-number ÷ ISO factor
$$ \text{Guide Number} = \,\frac{\text{Shooting Distance}\times \text{f-number}}{\text{ISO factor}} $$
I'm assuming that the GN calculated via the equation above will be based on a flash that is firing at full power. Am I correct to assume that if I set the flash to fire at half power, that the guide number is effectively halved? i.e. the equation will now be:
Guide Number = Shooting Distance × f-number ÷ ISO factor * Power Level
$$ \text{Guide Number} = \,\frac{\text{Shooting Distance}\times \text{f-number}}{\text{ISO factor}} \times \text{Power Level}$$
where power level can be the following 1, 1/2, 1/4, 1/8, ... 1/64, etc?
