FFT @ long period measure

I collected TEM images regarding a semicristalline structure and tried to measure the long period (cristalline lammelae+amorphous) with FFT signal (see the attached file). In the power spectrum I’ve measured the distance to the brightness point (r=12.4 nm). I’m not sure that value could be plausible. For your side that procedure can be correct ?
Many thx for your help in advantage