How to find the amplitude of a sinusoid in a signal?

Question

Algorithms

I calculate the amplitude of a useful sinusoid from the hardware-recorded data (noise, non-linear effects, sampling errors, etc. are present in the recording, a little bit of everything). After the window function and FFT, I normalize the data and find the maximum in the spectrum. To calculate the amplitude of a sinusoid, it is necessary to calculate the sum of the harmonics around the fundamental.
So the question is: how many harmonics to sum or how to calculate the width of the "summation area"? Tried a fixed number of harmonics, "as long as the harmonic is not below the threshold", summarized between local minima and a few more options. In all cases, the problem is expressed in the fact that for an integer and a non-integer number of waves in the recorded sample, the result is different (an error of tens of percent), apparently the spectrum "spreads" in different ways. How to do it right? What is the best window function to use?
Thank you.

Answer 1

https://en.wikipedia.org/wiki/Window_function#Flat... is the best window for finding the amplitude .