It goes from 0-1, so it should be easy to calculate based on something a little more tangible than 6" screen.
The PocketBook Touch HD 3 says it has a resolution of 1072 × 1448. With a diagonal of 6 inch (15.24 cm) that means 9.07 cm × 12.25 cm.
0.4/9.07 = 0.044
0.5/12.25 = 0.041
Practically speaking less than half a cm seems a bit small though. Might be hard to use.
