The guide number is inversely proportional to the power squared. This is due to the way that light intensity diminishes with distance, at twice the distance light is spread over four times the area, so each bit of that area receives 1/4 of the light.

So the actual formula needs to take into account the square root of the power level:

$$ \text{Guide Number} = \,\frac{\text{Shooting Distance}\times \text{f-number}\times \sqrt{\text{Power Level}}}{\text{ISO factor}} $$

The guide number has the same inverse square relationship to the sensitivity, as detailed by the ISO factor, defined as follows:

$$ \text{ISO factor} = \sqrt{\text{ISO}\over 100} $$

Substituting and bringing sensitivity onto the top of the fraction gives a formula which you can simply plug numbers into:

$$ \text{Guide Number} = \text{Shooting Distance}\times \text{f-number}\times \sqrt{\frac{\text{Power Level}\times 100}{\text{ISO}}} $$