Is that calculator right? are you using it right. I don't understand the things anyway.
Using your figures and approximating a bit, diameter 2m area 3.12m^2. wind speed 11.8m/s.
Thrust on a solid disc is 1/2 x rho x A x V^2.
Using 1.2 kg/m^3 for rho
thrust is .6 x 3.12 x 11.8 x 11.8 = 260N or about 57lb.
For a prop you can expect half that or even less. It is unlikely that you will be within rating of your alternator at 26 mph so you will likely furl earlier.
For safety think about 50 lb but expect less. If the tower can't stand this then you have a problem.
Flux