PSD scaling due to the windowing
The shortest answer to that question is to divide your PSD by the squared sum of window values $w_i$:

$$PSD(n) = \dfrac{2|X(n)|^2}{S^2} $$

where:

$n=0\ldots N/2 $

$S = \sum_{i=0}^{N-1}w_i$ 

You can read more on that topic in the following article:


  Heinzel G., et al - Spectrum and spectral density estimation by the
  DFT, including a comprehensive list of window functions and some new
  flat-top windows.