Finding Accurate Amplitude Using FFT

I have been working on code to use the FFT to find the amplitude of a signal. However, despite my best efforts to window I seem to get incorrect amplitudes. When I manually find Amplitude by inspecting the signal I get A = 63.7. When I use the following code I get A = 54.8. This is causing significant errors in my Bode plot.   I tried using a flat top window to maximize amplitude accuracy. My signal does not seem significantly noisy. I expected an amplitude much closer to the true value.   function Amplitude = Gaindb(x) if size(x, 2) > 1 x = x'; end x = x - mean(x); N = length(x); win = flattopwin(N); x = fft(x.*flattopwin(N)); Amplitude = 2*max(abs(x))/N; end