I would say between 20 & 50. Maybe closer to 20 end of scale (e.g. 30-ish). What type of lighting is in the room? The color temperature of the lighting is very important for these tests. They are designed for use under cool white lighting.
It looks darker than 20 but lighter than 50 to me.
I normally hold them against the white part next to the box that I think is closest rather than putting it on the circle. Just make sure you do it where there is good, white lighting. :)