RMS from calculation from PSD using Welch's method
Also, you can use the PSD for estimation of the RMS, the easier way for estimation of the RMS is a direct calculation. 

In case, you still want to use the PSD for estimation of the RMS, you should normalize the values by the square of the average value of the window. For the Hann window, the normalization factor will be $\frac{1}{4}$.

update
The question is a duplicate from RMS calculation in frequency domain after windowing