Sinusoid amplitude loss in power spectrum estimation using Pwelch method
I think your difficulty lies in having an overlap size of 0.

I'd recommend having an overlap that gives a Constant Overlap Add result when the windows are laid on top of each other.

For a rectangular window that is 0.
Hann it is 50% blocksize
Hamming seems to have many, but try 75%

Overlap values for different Windows

And don't forget that the PSD 'bin' represents all the content within that delta-f.  For example, if you had a signal at 100 Hz and 100.25 Hz, the 1 Hz delta-f wouldn't resolve them to 2 spikes .

RMS from a bin is sqrt(Area of bin).