Thanks for your expert adivce, Jan! And I kown you are the author of DGP, so I have another problem about DGP which is I got a high DGP (0.47) with the view facing the southern window when used Perez overcast sky with direct irradiation of 0 w/m^2 and diffuse irrdadiation of 228W/m^2.I suspect the validity of this result, because according to our life experience, dicomfort glare rarely happens under overcast sky. Moreover, I used solar total irradiance monitor to measure the outdoor direct and diffuse irradiation which are 57 W/m^2 and 251 w/m^2, and I also measured the vertical eye illuminance that is 2100lux with the view facing the southern window of a classroom. vertical eye illumiance of 2100 should appromaximately correspond to DGP of 0.35.
Below is the simulated HDR image:
