Human wind speed measurer on Sheffield Pike

I've tried this one before but last time I made the mistake of leaning forwards into the wind. Balancing on my toes made it harder to determine the exact pivot point. This is a refinement of the method as the heels make a clear pivot point. The red arrow is my weight which we'll say is 630N. Distance from red arrow to heels is roughly the length of my boot so let's say 0.3m. The clockwise moment is 630 x 0.3 = 189Nm. The blue arrow is the force of the wind on me. Newton's Second Law says that resultant force = rate of change of momentum. F=d(mv)/dt. Here we can write F=vdm/dt. We will assume that the air is stopped dead. The rate of change of mass is basically the mass of air that hits me and stops dead in one second. This is the mass of air contained in a volume as area Av as velocity tells you the distance for 1 second. My area = 1.8 x 0.3 = 0.54 square metres. The density of air is 1.2 kg per cubic metre so dm/dt = 1.2 x 0.54 x v = 0.65v kg per second. I will assume that the wind force acts half way up me so anticlockwise moment = 0.9 x 0.65 x v^2. Equating the moments, we get 0.9 x 0.65 x v^2 = 189. Wind speed v = 18 m/s. This is 65 km/h which is roughly 40 mph. It was a very windy day so that is not inconceivable.