Good catch, dana.
But, I don't understand why the 125% is only added to the basic 3.5 va/sq.ft. and not the additional 1 va/sq.ft.
I still would suggest the following...
50,000 x 4.5 = 225000
225,000 / 1920(80% of 120x20) = 117.19 (118 ckts.)
OR...
50,000 x 4.5 x 125% = 281,250
281,250 / 2400 (120x 20) = 117.19 (118 ckts.)
