What were actually doing is making illumination levels, just like a classical HDRI would contain.

The next settings for levels are slightly different. Move the left slider onward to 128, but also drag the maximum white level slider to about 180. This will dimm the bright white to a light grey, needed to cover the illumination of the HDRI completely. Like this:

Now, the wall and window look like the picture above. Save it as 7.psd. We now have the mid-illumination and three under exposed values (4.psd and 5, 6 and 7.psd). We just need three overexposed documents and we’re almost ready for a final detail. Set the right slider to respectively 212, 170 and 141 and save the results as 1.psd, 2.psd and 3.psd .

To see if there is a logical order of grey values and document naming.