Looks like the calculator is simply adding half the gust factor to the steady wind, and then calculating the crosswind component of the result. This means that the crosswind component at touchdown could be greater than you think.
The basic calculation of crosswind component is: Wind Speed x Cosine of the angle between the wind and the runway.
On this basis a good rule of thumb is: 15 degrees = 1/4 of the wind, 30 degrees =1/2 the wind, 45 degrees = 3/4 of the wind, and anything over 60 degrees = all of the wind as a crosswind component.
MJ
