This is similar to an Ektacolour printing guide tool that I have from the mid 80's, and it is potentially very useful if you print RA-4.
I use my rig when old papers have drifted beyond the range of my anaylser to recommend the filtration solution.
You hold the diffuser under the lens as you make the exposure. It works on the theory that shots taken in nature (i.e. not a picture of an all red painted room) integrate to a neutral grey.
You put the transparency on the colour paper as you make the diffused enposure, and process. Once the test print is dry you use the hex tool of various grey denisties to pick out which part of the cube image is closet to neutral grey of varied density.
Then you compare the 'right' density to the series of dots along the bottom of the transparency. The dot text tells you the corected exposure time/aperture adjustment, and the cube showing NG shows the corrected filter pack. Changing the filter pack may change the tiem/aperture as well. You have to do some calculations.
The second print should come out very close to the centre of the cube for nutral grey, and may need a 5 or 10cc correction, and a small aperture correction. After that, any picture taken on the same film, with the same lighting characteristics will use the test print determined filtration and aperture/time. You make aperture/time adjustments as you change the image size only, and these can be calcualted so as to do a blow up with no need to test print.