Finding phase angle via FFT
Since the length of your FFT is exactly one period of your sinusoidal input signal (N = Fs/F0) , the signal will appear in bin 1 (the one above DC) of the FFT result (the conjugate mirror is redundant for strictly real input).  To estimate the phase of the sinusoid of frequency F0, referenced to the start of the data window, just compute the atan2() of the signed imaginary and real components from that single complex bin 1.  No need to abs() components or sum or window or estimate the amplitude or examine any other bin.  Unless you want to sanity check the magnitude of bin 1 to make sure it isn't zero or just numerical noise.