This sensor isn't perfectly linear, but you can get close. The key is in the second graph. You can see that there is an almost linear relationship between 80cm and 7cm.
V = 20(1/d) + 0.2 (from reading the graph - you can probably get a bit more accurate than this)
(1/d) = (V-0.2) / 20
d = 20 / (V-0.2)
This won't hold closer than 7cm because it's not linear anymore.